Thymidine 5’-triphosphate

Ralitrexed is a folate analog with greater selectivity. It easily crosses the cell membrane and undergoes polyglutamation. Within tissues, ralitrexed may be stored up to 29 days. It directly inhibits thymidylate synthase, the key enzyme for synthesizing thymidine triphosphate (TTP). The drug has been described to induce apoptosis in tumor cells. Ralitrexed is used for the treatment of colon carcinomas. 

By this method, for instance, thymidine triphosphate (p3T) has been synthesized.1591... 

The answer is b. (Hardman, p 1203.) Trifluridine inhibits viral activity in HSV types 1 and 2, CMV, vaccinia, and perhaps adenovirus. It acts as a viral DNA synthesis inhibitor by irreversibly blocking thymidylate synthetase. Trifluridine triphosphate is a competitive inhibitor of thymidine triphosphate accumulation into DNA It is used in the treatment of primary keratoconjunctivitis and recurrent epithelial keratitis caused by HSV 1 and 2. 

D-Arabinofuranosyladenine, presumably as the triphosphate, inhibits the incorporation of precursors into DNA [154], and the triphosphate inhibits non-competitively the incorporation of thymidine triphosphate into DNA in extracts of ascites cells, suggesting a direct interaction with DNA polymerase, possibly at an allosteric site [155]. Other studies have confirmed the inhibition of DNA synthesis catalyzed by DNA polymerase [335, 335a]. 

Deoxyrihonucleotides are generally formed by reduction of ribonucleoside diphosphates. This involves a series of redox reactions in which NADP+ and FAD play a role (see Section 15.1.1), with a subsequent electron transport chain. DNA contains thymine rather than uracil, so thymidine triphosphate (dTTP) is a requirement. Methylation of dUMP to dTMP is a major route to thymine nucleotides, and is dependent upon N, A °-methylenetetrahydrofolate as the source of the methyl group (see Box 11.13). 

Trifluridine (trifluorothymidine) is a fluorinated pyrimidine nucleoside that inhibits viral DNA synthesis in HSV-1, HSV-2, CMV, vaccinia, and some adenoviruses. It is phosphorylated intracellularly by host cell enzymes, and then competes with thymidine triphosphate for incorporation by the viral DNA polymerase (Figure 49-3). Incorporation of trifluridine triphosphate into both viral and host DNA prevents its systemic use. Application of a 1% solution is effective in treating keratoconjunctivitis and recurrent epithelial keratitis due to HSV-1 or HSV-2. Cutaneous application of trifluridine solution, alone or in combination with interferon alfo, has been used successfully in the treatment of acyclovir-resistant HSV infections. 

Murray and Friedrichs (80) have obtained a 5 -nucleotidase from a particulate fraction of Ehrlich ascites tumor cells using deoxycholate. The relative rates of hydrolysis of 5 -UMP, 5 -AMP, 5 -CMP, 5 -GMP, and 5 -IMP are 129, 100, 93, 83, and 79, respectively. Adenosine and thymidine triphosphate are competitive inhibitors of 5 -AMP hydrolysis... 

After E. coli pyrophosphatase and 32Pj were mixed with the appropriate substrates and purified enzymes at pH 8.5, reduction of TPN+ proceeded with eventual trapping of 10% of the 3ZPi as thymidine triphosphate (TP3ZP3ZP). The net stoichiometry of the coupled reactions was 3ZP, —0.51 /xmole TP3ZP3ZP, +0.27 /xmole TDP-glucose, —0.26 /xmole and TPNH, +0.21 /xmole 12). 

Since DNA contains thymine (5-methyluracil) as a major base instead of uracil, the synthesis of thymidine monophosphate (dTMP, or thymidylate) is essential to provide dTTP (thymidine triphosphate), which is needed for DNA replication together with dATP, dGTP, and dCTP. 

Thymine Deoxyribose Thymidine Thymidylic acid thymidine monophosphate (dTMP) Thymidine diphosphate (dTDP) Thymidine triphosphate (dTTP)... 

Cells that have been preincubated with deoxyuridine, then exposed to [ H] thymidine, incorporate little or none of the labeled material into DNA. This is because of both dilution of the labeled material in the larger intracellular pool of newly synthesized TMP and also inhibition of thymidylate kinase by thymidine triphosphate. 

Fig. 3.5 (a) Spectral changes from NS-GQ before and after incubation in buffer, (b) Spectra of thymidine triphosphate, Guanidine triphosphate, and thrombin... 

TPP thiamine pyrophosphate Trp tryptophan TTP thymidine triphosphate Tyr tyrosine U uracil... 

Like idoxuridine. the antiviral mechanism of trifluridinc involves inhibition of viral DNA synthesis. Trifluridinc monophosphate is an irreversible inhibitor of thymidylate synthetase, and the biologically generated triphosphate competitively inhibits thymidine triphosphate incorporation into DNA by DNA polymera.se. In addition, trilluridine in its triphosphate form is incorporated into viral and cellular DNA. creating fragile, poorly functioning DNA. 

Another characteristic of competitive inhibition is structural similarity between substrate and competitive inhibitor. The similarity allows the inhibitor to bind in the active site of the enzyme. For example, the enzyme dihydrofolate reductase (DHFR) is subject to competitive inhibition by methotrexate and other compounds. Methotrexate is used in cancer chemotherapy because DHFR is required for the synthesis of thymidine triphosphate, a specific precursor of DNA (Fig. 8.8). The normal substrate for DHFR is folate and methotrexate is closely related (Fig. 8.2). 

Capecitabine is a pyrimidine analog. It is an oral systemic prodrug that is enzymatically converted to 5-fluorouracil (5-FU). Healthy and tumor cells metabolize 5-FU to 5-fluoro-2-deoxyuridine monophosphate (FdUMP) and 5-flu-orouridine triphosphate (FUTP). These metabolites cause cell injury by two different mechanisms. First, they inhibit the formation of thymidine triphosphate, which is essential for the synthesis of DNA. Second, nuclear transcriptional enzymes can mistakenly incorporate FUTP during the synthesis of RNA. This metabolic error can interfere with RNA processing and protein synthesis. Capecitabine is indicated in the treatment of resistant metastatic breast cancer alone or in combination with docetaxel, and colorectal cancer. 

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