The influence of 4-thiouridine labeling on pre-mRNA splicing outcomes

Metabolic labeling is a widely used tool to investigate different aspects of pre-mRNA splicing and RNA turnover. The labeling technology takes advantage of native cellular machineries where a nucleotide analog is readily taken up and incorporated into nascent RNA. One such analog is 4-thiouridine (4sU). Previous studies demonstrated that the uptake of 4sU at elevated concentrations (>50μM) and extended exposure led to inhibition of rRNA synthesis and processing, presumably induced by changes in RNA secondary structure. Thus, it is possible that 4sU incorporation may also interfere with splicing efficiency. To test this hypothesis, we carried out splicing analyses of pre-mRNA substrates with varying levels of 4sU incorporation (0-100%). We demonstrate that increased incorporation of 4sU into pre-mRNAs decreased splicing efficiency. The overall impact of 4sU labeling on pre-mRNA splicing efficiency negatively correlates with the strength of splice site signals such as the 3' and the 5' splice sites. Introns with weaker splice sites are more affected by the presence of 4sU. We also show that transcription by T7 polymerase and pre-mRNA degradation kinetics were impacted at the highest levels of 4sU incorporation. Increased incorporation of 4sU caused elevated levels of abortive transcripts, and fully labeled pre-mRNA is more stable than its uridine-only counterpart. Cell culture experiments show that a small number of alternative splicing events were modestly, but statistically significantly influenced by metabolic labeling with 4sU at concentrations considered to be tolerable (40 μM). We conclude that at high 4sU incorporation rates small, but noticeable changes in pre-mRNA splicing can be detected when splice sites deviate from consensus. Given these potential 4sU artifacts, we suggest that appropriate controls for metabolic labeling experiments need to be included in future labeling experiments.

Why Does the Type of Halogen Atom Matter for the Radiosensitizing Properties of 5-Halogen Substituted 4-Thio-2'-Deoxyuridines?

Radiosensitizing properties of substituted uridines are of great importance for radiotherapy. Very recently, we confirmed 5-iodo-4-thio-2'-deoxyuridine (ISdU) as an efficient agent, increasing the extent of tumor cell killing with ionizing radiation. To our surprise, a similar derivative of 4-thio-2'-deoxyuridine, 5-bromo-4-thio-2'-deoxyuridine (BrSdU), does not show radiosensitizing properties at all. In order to explain this remarkable difference, we carried out a radiolytic (stationary and pulse) and quantum chemical studies, which allowed the pathways to all radioproducts to be rationalized. In contrast to ISdU solutions, where radiolysis leads to 4-thio-2'-deoxyuridine and its dimer, no dissociative electron attachment (DEA) products were observed for BrSdU. This observation seems to explain the lack of radiosensitizing properties of BrSdU since the efficient formation of the uridine-5-yl radical, induced by electron attachment to the modified nucleoside, is suggested to be an indispensable attribute of radiosensitizing uridines. A larger activation barrier for DEA in BrSdU, as compared to ISdU, is probably responsible for the closure of DEA channel in the former system. Indeed, besides DEA, the XSdU anions may undergo competitive protonation, which makes the release of X- kinetically forbidden.

5-Iodo-4-thio-2'-Deoxyuridine as a Sensitizer of X-ray Induced Cancer Cell Killing

Nucleosides, especially pyrimidines modified in the C5-position, can act as radiosensitizers via a mechanism that involves their enzymatic triphosphorylation, incorporation into DNA, and a subsequent dissociative electron attachment (DEA) process. In this paper, we report 5-iodo-4-thio-2'-deoxyuridine (ISdU) as a compound that can effectively lead to ionizing radiation (IR)-induced cellular death, which is proven by a clonogenic assay. The test revealed that the survival of cells, pre-treated with 10 or 100 µM solution of ISdU and exposed to 0.5 Gy of IR, was reduced from 78.4% (for non-treated culture) to 67.7% and to 59.8%, respectively. For a somewhat higher dose of 1 Gy, the surviving fraction was reduced from 68.2% to 54.9% and to 40.8% for incubation with 10 or 100 µM ISdU, respectively. The cytometric analysis of histone H2A.X phosphorylation showed that the radiosensitizing effect of ISdU was associated, at least in part, with the formation of double-strand breaks. Moreover, the cytotoxic test against the MCF-7 breast cancer cell line and human dermal fibroblasts (HDFa line) confirmed low cytotoxic activity of ISdU. Based on the results of steady state radiolysis of ISdU with a dose of 140 Gy and quantum chemical calculations explaining the origin of the MS detected radioproducts, the molecular mechanism of sensitization by ISdU was proposed. In conclusion, we found ISdU to be a potential radiosensitizer that could improve anticancer radiotherapy.

5-Nitro-2-pyridyl-1-thioglucosides: application in synthesis of analogues of glycosyltransferases natural substrates

5-Nitro-2-pyridyl-1-thioglucosides were used in synthesis of complex uridine derivatives (13-16) in two different sequences of reactions. In one route, the first step was glycosylation of selectively protected 5-nitro-2-pyridyl-1-thioglucoside 1 with two different glycosyl donors (5 or 6), next, the nitro group in aglycone of obtained disaccharides 7 or 8 was reduced and then obtained products 9 or 10 were condensed with uridine derivatives 3 or 4 using DMT-MM as condensing agent under microwave irradiation. In the second route, condensation and glycosylation reactions were applied in reverse order. As it turned up, a sequence of reactions affected the yield of final glycoconjugates 13-16 and depended on the type of uridine derivatives used.

A forward-genetic screen and dynamic analysis of lambda phage host-dependencies reveals an extensive interaction network and a new anti-viral strategy

Latently infecting viruses are an important class of virus that plays a key role in viral evolution and human health. Here we report a genome-scale forward-genetics screen for host-dependencies of the latently-infecting bacteriophage lambda. This screen identified 57 Escherichia coli (E. coli) genes—over half of which have not been previously associated with infection—that when knocked out inhibited lambda phage's ability to replicate. Our results demonstrate a highly integrated network between lambda and its host, in striking contrast to the results from a similar screen using the lytic-only infecting T7 virus. We then measured the growth of E. coli under normal and infected conditions, using wild-type and knockout strains deficient in one of the identified host genes, and found that genes from the same pathway often exhibited similar growth dynamics. This observation, combined with further computational and experimental analysis, led us to identify a previously unannotated gene, yneJ, as a novel regulator of lamB gene expression. A surprising result of this work was the identification of two highly conserved pathways involved in tRNA thiolation—one pathway is required for efficient lambda replication, while the other has anti-viral properties inhibiting lambda replication. Based on our data, it appears that 2-thiouridine modification of tRNA^Glu, tRNA^Gln, and tRNA^Lys is particularly important for the efficient production of infectious lambda phage particles.

In this study, we took advantage of a new genetic resource for E. coli mutants to screen for previously undiscovered lambda phage host-dependencies. We then assessed the dynamics of infection in these different E. coli mutants and applied a mathematical model of infection in an attempt to further classify the role of these novel interactions. This model-driven approach to biological discovery led us to identify the previously uncharacterized gene yneJ as a regulator of lamB gene expression. In addition, we identified two highly conserved pathways involved in post-transcriptional modification of tRNA—one pathway was required for efficient lambda replication, while the other has anti-viral properties inhibiting lambda replication. This finding is important as it illustrates a new potential anti-viral strategy that could be applied broadly to other viruses.

Antiproliferative effect of 4-thiouridylate on OCM-1 uveal melanoma cells

PURPOSE

Brachytherapy is a well-established, effective treatment for uveal melanoma with a failure rate of 15%. The fatal consequence of unsuccessful treatments offers reason for improvement of the method. The authors propose using an apoptosis inducing agent locally, concomitantly with the well-established therapy, to sensitize the tumor cells. The authors propose a new nontoxic moderately active apoptosis inducing agent, 4-thio-uridylate (s4UMP), for this purpose.

METHODS

OCM-1 uveal melanoma cells were treated with various concentrations of s4UMP and its effect was monitored by measuring the cell viability (MTT assay). The following apoptosis detecting methods were performed to reveal the mechanism of decreased cell viability: light microscopy, DNA fragmentation assay, determination of caspase 9 activity, and FACS analysis.

RESULTS

The viability of uveal melanoma cells was decreased by 32%, 40%, and 9% after 24, 48, and 72 hours of treatment with 10 microg/mL (30 microM) s4UMP. The effect was not dose dependent; it rather followed a saturation-type inhibition and the cells at lower drug concentration recovered after 72 hours. Characteristic apoptotic cell morphology and DNA fragmentation was detected in treated cells. The caspase-9 was activated upon treatment showing maximal activity at 48 hours suggesting the induction of apoptosis. The annexin binding activity further verified the apoptogenic activity of s4UMP.

CONCLUSIONS

Uveal melanoma, more than other solid tumors, is resistant to most of the chemotherapeutic protocols as indicated by the high mortality rate of metastatic disease. The authors showed that s4UMP, a naturally occurring nucleotide, could induce apoptosis in uveal melanoma cells, suggesting a potential supplementary therapeutic application of the compound.

Effects of uridine, isomatitol and 4-thiouridine on in vitro cell adhesion and in vivo effects of 4-thiouridine in a lung inflammation model

Since leukocyte adhesion to endothelial cells is crucial for extravasation of leukocytes to sites of inflammation, inhibition of cell-cell adhesion has been suggested as a means to achieve selective modulation of the immune system. We have, using a static in vitro adhesion assay involving adhesion of granulocytes to tumor necrosis factor alpha (TNFalpha)-stimulated human umbilical vein endothelial cells (HUVEC), found three substances--uridine, isomaltitol and 4-thiouridine-that, independently and significantly, reduced leukocyte adhesion by approximately 30-65%. 4-Thiouridine was also tested in an in vivo model of Sephadex (SDX)-induced lung inflammation with Sprague-Dawley rats. Intratracheal instillation of Sephadex (5 mg/kg) alone resulted in a dramatic increase in lung edema and total leukocyte count after 24 h. A differential count of bronchoalveolar lavage (BAL) cells indicated an increased influx of macrophages, eosinophils and neutrophils. Co-administration of 4-thiouridine significantly reduced lung edema by 38%. There was also a significant reduction of the total leukocyte count by 58%. The differential leukocyte count indicated that eosinophil influx alone was reduced by 70%. After Sephadex challenge, we found elevated levels of TNFalpha--an important inflammatory mediator--in the bronchoalveolar lavage fluid (BALF). TNFalpha levels were significantly reduced by more than 80% by co-administration of 4-thoiuridine. These results suggest that uridine, isomaltitol and, especially, 4-thiouridine affect adhesion between leukocytes and activated endothelium, and warrant further in vitro and in vivo studies.

4-Thio-5-bromo-2′-deoxyuridine: chemical synthesis and therapeutic potential of UVA-induced DNA damage

4-Thio-5-bromo-2'-deoxyuridine (3a) is prepared from 5-bromo-2'-deoxyuridine (BrdU) and its key properties are explored. The thionucleoside (3a) can react readily with monobromobimane and produces high fluorescence. 3a has UV maximum absorption at 340 nm and can be incorporated into cellular DNA. The cells containing 3a become sensitive to UVA light, offering therapeutic potential for UVA-induced cell killing.

4-thio-U cross-linking identifies the active site of the VS ribozyme

To identify nucleotides in or near the active site, we have used a circularly permuted version of the VS ribozyme capable of cleavage and ligation to incorporate a single photoactive nucleotide analog, 4-thio- uridine, immediately downstream of the scissile bond. Exposure to UV light produced two cross-linked RNAs, in which the 4-thio-uridine was cross-linked to A756 in the 730 loop of helix VI. The cross-links formed only under conditions that support catalytic activity, suggesting that they reflect functionally relevant conformations of the RNA. One of the cross-linked RNAs contains a lariat, indicative of intramolecular cross-linking in the ligated RNA; the other is a branched molecule in which the scissile phosphodiester bond is cleaved, but occupies the same site in the ribozyme-substrate complex. These are the two forms of the RNA expected to be the ground state structures on either side of the transition state. This localization of the active site is consistent with previous mutational, biochemical and biophysical data, and provides direct evidence that the cleavage site in helix I interacts with the 730 loop in helix VI.

Identification of active antiviral compounds against a New York isolate of West Nile virus

The recent West Nile virus (WNV) outbreak in the United States has increased the need to identify effective therapies for this disease. A chemotherapeutic approach may be a reasonable strategy because the virus infection is typically not chronic and antiviral drugs have been identified to be effective in vitro against other flaviviruses. A panel of 34 substances was tested against infection of a recent New York isolate of WNV in Vero cells and active compounds were also evaluated in MA-104 cells. Some of these compounds were also evaluated in Vero cells against the 1937 Uganda isolate of the WNV. Six compounds were identified to be effective against virus-induced CPE with 50% effective concentrations (EC50) less than 10 microg/ml and with a selectivity index (SI) of greater than 10. Known inhibitors of orotidine monophosphate decarboxylase and inosine monophosphate dehydrogenase involved in the synthesis of GTP, UTP, and TTP were most effective. The compounds 6-azauridine, 6-azauridine triacetate, cyclopententylcytosine (CPE-C), mycophenolic acid and pyrazofurin appeared to have the greatest activities against the New York isolate, followed by 2-thio-6-azauridine. Anti-WNV activity of 6-azauridine was confirmed by virus yield reduction assay when the assay was performed 2 days after initial infection in Vero cells. The neutral red assay mean EC50 of ribavirin was only 106 microg/ml with a mean SI of 9.4 against the New York isolate and only slightly more effective against the Uganda isolate. There were some differences in the drug sensitivities of the New York and Uganda isolates, but when comparisons were made by categorizing drugs according to their modes of action, similarities of activities between the two isolates were identified.

Pyrimidinethione nucleosides and their deaza analogues

The methods of preparation, structure, chemical properties and synthetic potentiality of pyrimidinethione nucleosides and their deaza analogues are reported.

Synthesis and hypnotic activities of 4-thio analogues of N3-substituted uridines

Reaction of tri-O-acetyluridine (1) with benzyl bromide or 2-chloroacetophenone in the presence of K2CO3 gave the N3-substituted analogues 2a,c. Condensation of 1 with (+/-)-1-phenylethanol or 3,5-dimethylbenzyl alcohol using the Mitsunobu reaction also gave 2b,d in good yields. These compounds were allowed to react with Lawesson's reagent and were subsequently treated with ammonia to afford the 4-thiouracil derivatives 5a-d. Compounds 5a-c showed moderate hypnotic activity in mice. However, N3-(3,5-dimethyl)benzyl derivatives 3d, 5d were found to be almost inactive in this assay.

Translational misreading: a tRNA modification counteracts a +2 ribosomal frameshift

Errors during gene expression from DNA to proteins via transcription and translation may be deleterious for the functional maintenance of cells. In this paper, extensive genetic studies of the misreading of a GA repeat introduced into the lacZ gene of Escherichia coli indicate that in this bacteria, errors occur predominantly by a +2 translational frameshift, which is controlled by a tRNA modification involving the MnmE and GidA proteins. This ribosomal frameshift results from the coincidence of three events: (1) decreased codon-anticodon affinity at the P-site, which is caused by tRNA hypomodification in mnmE(-) and gidA(-) strains; (2) a repetitive mRNA sequence predisposing to slippage; and (3) increased translational pausing attributable to the presence of a rare codon at the A-site. Based on genetic analysis, we propose that GidA and MnmE act in the same pathway of tRNA modification, the absence of which is responsible for the +2 translational frameshift. The difference in the impact of the mutant gene on cell growth, however, indicates that GidA has at least one other function.

Mutation of Gln125 to Asn selectively abolishes the thymidylate kinase activity of herpes simplex virus type 1 thymidine kinase

The broad substrate specificity of herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) has provided the basis for selective antiherpetic therapy and, more recently, suicide gene therapy for the treatment of cancer. We have now constructed an HSV-1 TK mutant enzyme, in which an asparagine (N) residue is substituted for glutamine (Q) at position 125, and have evaluated the effect of this amino acid change on enzymatic activity. In marked contrast with wild-type HSV-1 TK, which displays both thymidine kinase and thymidylate kinase activities, the HSV-1 TK(Q125N) mutant was unable to phosphorylate pyrimidine nucleoside monophosphates but retained significant phosphorylation activity for thymidine and a series of antiherpetic pyrimidine and purine nucleoside analogs. The abrogation of HSV-1 TK-associated thymidylate kinase activity resulted in a 100-fold accumulation of the monophosphate form of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) in osteosarcoma cells transfected with the HSV-1 TK(Q125N) gene compared with osteosarcoma cells expressing wild-type HSV-1 TK. BVDU monophosphate accumulation gave rise to a much greater inhibition of cellular thymidylate synthase in HSV-1 TK(Q125N) gene-transfected cells than wild-type HSV-1 TK gene-transfected osteosarcoma tumor cells without significantly changing the cytostatic potency of BVDU for the HSV-1 TK gene-transfected tumor cells. Accordingly, the presence of the Q125N mutation in HSV-1 TK gene-transfected tumor cells was found to result in a multilog decrease in the cytostatic activity of those pyrimidine nucleoside analogs that in their monophosphate form do not have marked affinity for thymidylate synthase [i.e., 1-beta-D-arabinofuranosylthymine and (E)-5-(2-bromovinyl)-1-beta-D-arabinofuranosyluracil].

Arrangement of the central pseudoknot region of 16S rRNA in the 30S ribosomal subunit determined by site-directed 4-thiouridine crosslinking

The 16S rRNA central pseudoknot region in the 30S ribosomal subunit has been investigated by photocrosslinking from 4-thiouridine (s4U) located in the first 20 nt of the 16S rRNA. RNA fragments (nt 1-20) were made by in vitro transcription to incorporate s4U at every uridine position or were made by chemical synthesis to incorporate s4U into one of the uridine positions at +5, +14, +17, or +20. These were ligated to RNA containing nt 21-1542 of the 16S rRNA sequence and, after gel purification, the ligated RNA was reconstituted into 30S subunits. Long-range intramolecular crosslinks were produced by near-UV irradiation; these were separated by gel electrophoresis and analyzed by reverse transcription reactions. A number of crosslinks are made in each of the constructs, which must reflect the structural flexibility or conformational heterogeneity in this part of the 30S subunit. All of the constructs show crosslinking to the 559-562, 570-571, and 1080-1082 regions; however, other sites are crosslinked specifically from each s4U position. The most distinctive crosslinking sites are: 341-343 and 911-917 for s4U(+5); 903-904 (very strong), 1390-1397, and 1492 for s4U(+14); and 903-904 (moderate) for s4U(+17); in the 1070-1170 region in which there are different patterns for each s4U position. These results indicate that part of the central pseudoknot is in close contact with the decoding region, with helix 27 in the 885-912 interval and with part of domain III RNA. Crosslinking between s4U(+14) and 1395-1397 is consistent with base pairing at U14-A1398.

Anti-(herpes simplex virus) activity of 4'-thio-2'-deoxyuridines: a biochemical investigation for viral and cellular target enzymes

The antiviral activity of several nucleoside analogues is often limited by their rapid degradation by pyrimidine nucleoside phosphorylases. In an attempt to avoid this degradation, several modified nucleosides have been synthesized. A series of 4'-thio-2'-deoxyuridines exhibits an anti-[herpes simplex virus (HSV)] activity significantly higher (20-600 times) than that shown by the corresponding 4'-oxy counterpart. We investigated the mode of action of these compounds and we found that: (i) several 4'-thio-2'-deoxyuridines are phosphorylated to the mono- and di-phosphates by HSV-1 thymidine kinase (TK) more efficiently than their corresponding 4'-oxy counterpart; (ii) both are inhibitors of cellular thymidylate synthase; (iii) 4'-thio-2'-deoxyuridines are resistant to phosphorolysis by human thymidine phosphorylase; (iv) both 4'-oxy- and 4'-thio-2'-deoxyuridines are phosphorylated to deoxyribonucleotide triphosphate in HSV-1-infected cells and are incorporated into viral DNA; (v) 4'-thio-2'-deoxyuridines are better inhibitors than their 4'-oxy counterparts of [(3)H]thymidine incorporation in HSV-1-infected cells; (vi) 4'-thio-2'-deoxyuridines are not recognized by HSV-1 and human uracil-DNA glycosylases. Our data suggest that 4'-thio-2'-deoxyuridines, resistant to pyrimidine phosphorylase, can be preferentially or selectively phosphorylated by viral TK in HSV-infected cells, where they are further converted into triphosphate by cellular nucleotide kinases. Once incorporated into viral DNA, they are better inhibitors of viral DNA synthesis than their corresponding 4'-oxy counterpart, either because they are not recognized, and thus not removed, by viral uracil-DNA glycosylase, or because they preferentially interfere with viral DNA polymerase.

2'-Deoxy-5-ethyl-beta-4'-thiouridine inhibits replication of murine gammaherpesvirus and delays the onset of virus latency

The antiviral thionucleoside analogue 2'-deoxy-5-ethyl-beta-4'-thiouridine (4'-S-EtdU) was shown to be a more potent inhibitor of gammaherpesvirus infection than acyclovir. This compound inhibits replication of murine herpesvirus (MHV)-68 in the lungs of mice when given 3 days post-infection. However, as with other nucleoside analogues, it was unable to prevent the establishment of latency, despite delaying the onset of latent infection in the spleen. In contrast, virus persistence in the lung was inhibited following drug treatment, although persistence was re-established in mice when treatment was suspended after 12 days. These data suggest that 4'-S-EtdU is a highly effective inhibitor of murine gammaherpesvirus replication and as such provides a powerful tool to study the pathogenesis of this virus in vivo.

Comparative cytostatic activity of different antiherpetic drugs against herpes simplex virus thymidine kinase gene-transfected tumor cells

A series of selective antiherpetic compounds were found to exert pronounced cytostatic activity against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) thymidine kinase (TK) gene-transfected mammary carcinoma FM3A cells. Based on their potency and mechanism of cytostatic action, the antiherpetic compounds could be divided into two different classes. The first class encompasses (E)-5-(2-bromovinyl)-2'-deoxyuridine and structurally related analogues thereof [i.e., the cytosine derivative (E)-5-(2-bromovinyl)-2'-deoxycytidine and the 4'-thio derivative (E)-5-(2-bromovinyl)-2'-deoxy-4'-thiouridine]. These compounds are exquisitely cytostatic against FM3A/TK-/HSV-1 TK+ and FM3A/TK-/HSV-2 TK+ cells (50% inhibitory concentrations ranging from 0.047 to 0.001 microM) and inhibit tumor cell proliferation by inhibiting cellular thymidylate synthase. The second class consists of the acyclic guanosine derivatives penciclovir, buciclovir, and ganciclovir. These compounds are also more inhibitory to the HSV-1 TK or HSV-2 TK gene-transfected FM3A cells than to FM3A/0 or FM3A/TK- cells, but at concentrations that are higher than the concentrations at which the (E)-5-(2-bromovinyl)-2'-deoxyuridine derivatives proved to be inhibitory. These acyclic guanosine analogues appear to be targeted at the cellular DNA polymerase. From this study, (E)-5-(2-bromovinyl)-2'-deoxy-4'-thiouridine emerged as a promising candidate compound for the treatment of HSV-1 TK gene-transfected tumors in vivo, due to its metabolic stability (i.e., resistance to hydrolysis by thymidine phosphorylase).

2-Thio derivatives of dUrd and 5-fluoro-dUrd and their 5'-monophosphates: synthesis, interaction with tumor thymidylate synthase, and in vitro antitumor activity

A convenient synthesis of 5-fluoro-2-thiouracil (11) is based on hydrolytic deamination of 5-fluoro-2-thiocytosine (9). Lewis acid-catalyzed condensation of di-TMS-5-fluoro-2-thiouracil (13) or di-TMS-2-thiouracil (14) with 2-deoxy-3,5-di-O-p-toluyl-D-ribofuranosyl chloride (15) led to mixtures of the beta- and alpha-anomers of 3',5'-toluylated 2'-deoxy-5-fluoro-2-thiouridine (16 and 18) or 2'-deoxy-2-thiouridine (17 and 19), each of which was deblocked with MeOH-NH3 to give the desired free anomeric nucleoside pairs 1, 5 and 3, 7, respectively. These were selectively converted to the corresponding 5'-monophosphates 2, 6 and 4, 8, with the aid of the wheat shoot phosphotransferase system. Conformations of the nucleosides 1, 3, 5, 7 are deduced from 1H NMR spectra, and circular dichroism spectra for nucleotide anomeric pairs 2, 6 and 4, 8 are reported. Whereas beta-2-thio-dUMP (4) was a good substrate (Km approximately 10(-5) M), beta-5-fluoro-2-thio-dUMP (2) proved to be a potent competitive, slow-binding inhibitor (Ki approximately 10(-8) M) of the purified enzymes from Ehrlich ascites carcinoma and L1210 cells. The alpha-anomer 6 was a weak inhibitor, with Ki in the mM range, and its congener 8 hardly interacted with the enzyme. The beta-anomer 1 exhibited antitumor activity in a mouse leukemic cell line L5178Y (IC50 approximately 10(-6) M), hence 40-100-fold weaker than 5-fluoro-dUrd. Its alpha-anomer 5 was 10-fold less active, but exhibited at least 10-fold higher selectivity with respect to the tumor cells than the beta-anomer 1.

Synthesis and biological activity of 2'-deoxy-4'-thio pyrimidine nucleosides

2'-Deoxy-4'-thiocytidine (7 beta), 2'-deoxy-4'-thiouridine (9), and 4'-thiothymidine (10) have been synthesized and evaluated for cytotoxicity in vitro. All these compounds were cytotoxic to L1210, H-Ep-2, and CCRF-CEM cell lines. 4'-Thiothymidine was also active against herpes simplex 1 and human cytomegalovirus in cell culture.

Substitution of uridine in vivo by the intrinsic photoactivable probe 4-thiouridine in Escherichia coli RNA. Its use for E. coli ribosome structural analysis

In vivo incorporation of the uridine-photoactivable analogue, 4-thiouridine, into the ribosomal RNA of an Escherichia coli pyrD strain has been demonstrated. It is highly dependent on the exogenous uridine and 4-thiouridine concentrations as well as on temperature. We have defined conditions allowing the substitution of 13 +/- 2% of the uridine residues in bulk RNA by 4-thiouridine. On a high-Mg2+ sucrose gradient, 33 +/- 3% of ribonucleic particles sediment as 70S ribosomes, the remaining being in the form of non-associated 50S and 30S particles containing immature rRNA. The thiolated 70S ribosomes tolerate a 4-5% substitution level (40 thiouridine molecules/particle). Surprisingly, 3-4% of ribosomal proteins, about two protein molecules/particle, were spontaneously covalently bound to 4-thiouridine-substituted rRNA. Specific 366-nm photoactivation increased this proportion to 10-12%, i.e. up to six or seven ribosomal protein molecules/particle. The photochemical cross-linking proceeds with apparent first-order kinetics with a quantum yield close to 5 X 10(-3). Although extensive photodynamic breakage of rRNA occurs under aerobic conditions, both the kinetics and yield of ribosomal protein cross-linking were independent of oxygenation conditions. The thiolated (4.5%) 70S ribosomes allowed the poly(U)-directed poly(Phe)synthesis at 48% the control rate. Photoactivation decreased this activity to 28% and 10% when performed under nitrogen and in aerated conditions, respectively.

Synthesis of some new S-alkylated derivatives of 5-mercapto-2'-deoxyuridine as potential antiviral agents

A series of S-alkylated derivatives of 5-mercapto-2'-deoxyuridine have been prepared by alkylation of the preformed nucleoside. Two of these compounds, the S-propargyl and S-allyl derivatives, have shown significant antiviral activity against Herpes simplex type 1 in HeLa TK- cells but appear to be less effective in this assay system than some previously reported 5-substituted 2'-deoxyuridines.

[4-Thiouridine and photoprotection in Escherichia coli K 12]

A high level of protection is observed in the Escherichia coli K 12 strain AB 1157 rec A 1 nuv+ whose transfer RNA contains 4-thiouridine. In contrast, the photoprotection level is low and observed at higher doses in a strain which differs from the former by a single mutation, nuv-, (lack of 4-thiouridine). This nucleoside is therefore an important chromophore leading to photoprotection. This conclusion is corroborated by the similarity of the action spectra for 8-13 link formation in tRNA and for photoprotection.

Effects of 5-mercapto-2'-deoxyuridine on the incorporation of nucleosides into RNA and DNA in a primary lymphocyte culture system

The effects of 5-mercapto-2'-deoxyuridine (MUdr) on DNA synthesis in a primary murine spleen lymphocyte culture system stimulated by phytohemagglutinin (PHA) were studied. Inhibition of thymidine incorporation into acid insoluble nucleic acid material was 50% at 0.5 mM MUdR concentration, while inhibition of deoxyuridine incorporation into acid-insoluble nucleic acids was 50% at 0.01 mM MUdR. Time course studies, at 0.5 and 0.05 mM MUdR, showed that the magnitude of inhibition of incorporation for thymidine and deoxyuridine, respectively, increased from a time point after PHA stimulation when increased synthesis of thymidine kinase and thymidylate synthetase had leveled off. At 1 mM MUdR, total cellular DNA in cultures was decreased 43% at 42 hr after PHA stimulation. Neither the total number of cells nor the percentage of PHA-transformed cells was decreased in comparison to that of controls. MUdR therefore blocks the increase in DNA content of lymphocytes that is initiated during the S phase of the cell cycle. Millimolar levels of MUdR inhibited incorporation or uridine, adenosine, and cytidine into acid-insoluble material in pha-stimulated primary murine lymphocyte cultures. Total cellular RNA synthesis was inhibited at these levels of MUdR, with no differential effects on 4, 18, or 28 S RNA species observed. Uptake of these nucleosides into the total cellular acid-soluble material was not blocked. Uptake of different labeled nucleosides into cellular, acid-soluble pools occurs at different rates. Thus, choice of a suitable minimum pulse time to achieve saturation for different labeled nucleosides must relate to this consideration. Thymidine kinase from whole-cell sonic extracts of PHA-stimulated lymphocytes was inhibited 65% by 1 mM MUdR at 24 and 48 hr after stimulation. Uridine kinase extracted from the PHA-stimulated cells was also significantly inhibited by 1mM MUdR at 24 hr (56%). Exogenous guanosine incorporation into lympohcyte acid-insoluble material is increased by MUdR. This increased utilization of exogenous nuceloside is apparently the result of MUdR inhibition of conversion of adenosine to guanine nucleotides within the lymphocytes and a consequent diminution of the total intracellular guanine nucleotide pool size. The active inhibitory compound is the deoxyribonucleoside or deoxyribonucleotide. Comparison with the riboside analog 5-mercaptouridine showed that MUdR was a more efficient inhibitor of nucleoside incorporation.

Inhibition of turnip yellow mosaic virus synthesis by pyrimidine analogues

The pyrimidine analogues 2-thiouracil, 2-thiouridine, 6-azauracil and 6-azauridine all inhibited the synthesis of turnip yellow mosaic virus (TYMV) and increased the synthesis of empty virus protein shells in infected Chinese cabbage leaf discs. Uracil and uridine reversed these effects. 2-Thiouracil also reduced the UTP pool in TYMV infected leaf discs. The results are consistent with the suggestion that these analogues or their in vivo derivatives affect virus synthesis by inhibiting the biosynthesis of uridylic acid, possibly by inhibiting orotidylic acid decarboxylase.

Synthesis and biological activity of 5-fluoro-4'-thiouridine and some related nucleosides

The synthesis of a series of 4'-thio-5-halogenopyrimidine nucleosides, including the 5-fluoro, chloro, bromo and iodo derivatives, has been carried out by condensation of the 2,4-bis-O-trimethylsilyl derivatives of the corresponding pyrimidine bases with the protected 4-thio-D-ribofuranosyl chloride. Among these, the alpha and beta anomers of 4'-thio-5-fluorouridine inhibited the growth of leukemia L1210 cells at concentrations of 4 x 10(-7) and 2 x 10(-7) M, respectively, and that of S. faecium at 4 x 10(-9) and 6 x 10(-10) M, respectively. These compounds retained marked activity against strains of S. faecium resistant to 10(-3) M 5-fluorouracil or 5-fluorouridine. As determined in S. faecium cultures, 4'-thio-5-fluorouridine decreased the total protein content of the cells more markedly than it did their RNA or DNA content. X-Ray crystallography showed that substitution of sulfur for the oxygen in the carbohydrate ring markedly changes the conformation of that moiety.