Thymidine nucleotides

With the aid of cytosine permease, flucytosine reaches the fungal cell where it is converted by cytosine deaminase into 5-fluorouracil [51-21-8]. Cytosine deaminase is not present in the host, which explains the low toxicity of 5-FC. 5-Fluorouracil is then phosphorylated and incorporated into RNA and may also be converted into 5-fluorodeoxyuridine monophosphate, which is a potent and specific inhibitor of thymidylate synthetase. As a result, no more thymidine nucleotides are formed, which in turn leads to a disturbance of the DNA-synthesis. These effects produce an inhibition of the protein synthesis and cell repHcation (1,23,24). 5-Fluorouracil caimot be used as an antimycotic. It is poorly absorbed by the fungus to begin with and is also toxic for mammalian cells. 

Shoji A, Hasegawa T, Kuwahara M et al (2007) Chemico-enzymatic synthesis of a new fluorescent-labeled DNA by PCR with a thymidine nucleotide analogue bearing an acridone derivative. Bioorg Med Chem Lett 17 776-779... 

These one-carbon groups, which are required for the synthesis of purines, thymidine nucleotides and for the interconversion some amino acids, are attached to THF at nitrogen-5 (N5), nitrogen-10 (N10) or both N5and N10. Active forms of folate are derived metabolically from THF so a deficiency of the parent compound will affect a number of pathways which use any form of THF. 

Single zinc(II)-cyclenes had been known to bind to uridine and thymidine nucleotides by the specific zinc(II)-imide N" bonding. The repeated binding motif was found to selectively bind to a dinucleotide dTpdT or a trinucleotide dTpdTpdT, according to the number of zinc(II)-cyclene units. 

The inability to absorb Vitamin B12 occms in pernicious anemia. In pernicious anemia intrinsic factor is missing. The anemia results from impaired DNA synthesis due to a block in purine and thymidine biosynthesis. The block in nucleotide biosynthesis is a consequence of the effect of vitamin B12 on folate metabolism. When vitamin B-12 is deficient essentially all of the folate becomes trapped as the N -methyltetrahydrofolate derivative as a result of the loss of functional methionine synthase. This trapping prevents the synthesis of other tetrahydrofolate derivatives. required for the purine and thymidine nucleotide biosynthesis pathways. 

Synthesis of Thymidine nucleotides first requires deoxyribonucleotide synthesis. The enzyme responsible for this step is Ribonucleotide Reductase. This enzyme acts on oxynucleotides in their diphosphate form. Thioredoxin, a small protein, is oxidized as the 2 hydroxyl group on the ribose ring is reduced. Oxidized Thioredoxin (S-S) is then reduced by FADH2 and NADPH. The products are the respective deoxynucleotide diphosphates which are further phosphorylated and then used for DNA synthesis. 

The exception dUDP is first converted to thymidine nucleotide via... 

Inhibition of the Reductase affects folate metabolism leading to decreased glycine formation from serine and decreased purine synthesis which requires CH3-THF. To facilitate normal cells folinic acid (Leucovorin) is given along with methotrexate. This acid aids normal cells by its conversion to the coenzyme of Thymidyiate S)mthetase, thus bypassing the block. Since the thymidine nucleotide requirements of rapidly proliferating cells are much greater than for quiescent cells folinic acid cannot meet the demands of the cancer cells. 

UMP is the parent compound in the synthesis of cytidine and deoxycytidine phosphates and thymidine nucleotides (which are deoxyribonucleotides). 

What are called antifolate drugs pertain in general to blocking the biosynthesis of purines and pyrimidines, the heterocyclic bases used in the further synthesis of DNA and RNA, where folic acid is required as a coenzyme (or vitamin) for the enzyme dihydrofolate reductase. The previously mentioned compound called methotrexate or amethopterin (4-amino-A °-methyl folic acid), being a structural analog of folate or folic acid, locks up the enzyme dihydrofolate reductase, which in turn blocks the synthesis of a thymidine nucleotide necessary for cell division. 

Cytosolic thymidine kinase salvages exogenous thymidine extremely efficiently. Experiments with radiolabeled precursors show that dTTP derived from salvage synthesis is usually incorporated into DNA in preference to thymidine nucleotides generated by de novo synthesis. 

To avoid incorporating uridines into DNA, cells have developed a rather a simple mechanism. The enzyme, dUXPase, converts dUTP (a substrate for incorporation into DNA) to dUMP (not a substrate for incorporation into DNA), and also provides a route to synthesis of thymidine nucleotides because the dUMP, in turn, is converted first to dTMP then to dTTP (Figure 22.17). 

The synthesis of deoxyuridine, cytidine, deoxycytidine and thymidine nucleotides from UMP (Fig. 6.13) involves three reactions CTP synthetase, ribonucleotide reductase, and thymidylate synthase (80). The first enzyme converts UTP into CTP and the second catalyzes the conversion of CDP, UDP, ADP and GDP into their respective deoxyribonucleotides. The last enzyme, thymidylate synthase, catalyzes the reductive methylation of deoxyUMP at the C-5 position giving deoxyTMP. The human enzyme has been extensively studied as it is a target enzyme in cancer chemotherapy. Besides these three enzymes, two other enzymes are involved in pyrimidine nucleotide synthesis and interconversion. DeoxyCMP deaminase converts deoxyCMP into deoxyUMP and deoxyUTP triphosphatase converts deoxyUTP into deoxyUMP. Giardia lamblia, and Trichomonas vaginalis lack both ribonucleotide reductase and thymidylate synthase and... 

The effect of acronycine (30) on the growth of cultured cells and of nucleoside uptake and incorporation into nucleic acids by the cells has been studied. It was found that, although acronycine does not inhibit nucleic acid synthesis, it does inhibit the accumulation of extracellular uridine and thymidine nucleotides in the precursor pool. Oral administration of dubamine to mice shows no apparent toxic elTects. ... 

Thymidylate synthase methylated the deoxyuridylate into thymidylate using 5,10-methylenetetrahydrofolate as a cofactor. This reaction is the rate-limiting step in the de novo synthetic pathway to thymidine nucleotide. Antitumoral effects are obtained with antifolate compounds. ... 

Although in the original method (Breitman, 1963) the counting efficiency was only 2% for (%)-labelled thymidine nucleotides, the technique has also been adopted for assaying thymidine kinase in developing rat cerebellum (Weichsel, 1974). As can be expected, their values are low in comparison with those works in which thymidine nucleotides were separated by thin-layer chromatography (Yamagami et al., 1972). Further-... 

The unconverted [6- H]thymidine was fully recovered in the effluent and water wash. Next, the absorbed thymidine nucleotides were eluted with 30 ml of 2 M HCl. The eluate was evaporated to dryness, if necessary, and redissolved in distilled water before the estimation of radioactivity. 

B. Method 2, The thymidine nucleotides from the supernatant of the enzyme reaction were separated using the method of Weichsel (1974). Briefly, the same aliquots of the supernatant as in Method 1 were applied onto Whatman DE-81 disks on a Millipore filter holder fitted into a suction flasks. The disks were each washed twice with 10 ml of 1 mM ammoniiam formate and 10 ml of distilled water and once with 10 ml of absolute ethanol. After drying the disks were placed in a counting vial with 10 ml of scintillation fluid. Radioactivity was measured in a Nuclear Chicago Mark 2 Scintillation Spectrometer. ... 

TABLE I. Comparison of Counting / E/Thymidine Nucleotides with Different Methods... 

Mechanism of action Fluorouracil is an analogue of uracil. After entry into cells it undergoes conversion to active metabolites fluorodeoxyuridine monophosphate (FdUMP), fluorodeoxyuridine triphosphate (FdUTP) and fluorouridine triphosphate (FUTP). FdUMP directly inhibits thymidy-late synthetase, reducing the availability of thymidine nucleotides, which are required for DNA synthesis, until new enzyme can be synthesized. FUTP is incorporated into RNA and causes impaired RNA processing and functioning, which disrupts cellular metabolism and viability [71 ]. 

These studies indicate a specific depletion of thymidine nucleotides appear to be involved in fragile chromosome expression, whereas the restriction of amino acids or purines have no apparent affect. 

Receptor 112 also has the ability, of the Zn + complex of [12]aneN4, to bind thymidine nucleotides previously discussed in such a way that they can stack upon... 

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