Pyrimidine salvage

There are basically two types of salvage. The first involves attachment of the base to PRPP with the formation of pyrophosphate. This pathway is available for salvage of purines and uracil but not for cytosine or thymine. The other pathway involves attachment of the base to ribose 1-phosphate, which occurs to some extent for most of the purines and pyrimidines. This second pathway requires the presence of specific 

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While mammahan cells reutilize few free pyrimidines, salvage reactions convert the ribonucleosides uridine and cytidine and the deoxyribonucleosides thymidine and deoxycytidine to their respective nucleotides. ATP-dependent phosphoryltransferases (kinases) catalyze the phosphorylation of the nucleoside diphosphates 2 "-de-oxycytidine, 2 -deoxyguanosine, and 2 -deoxyadenosine to their corresponding nucleoside triphosphates. In addition, orotate phosphoribosyltransferase (reaction 5, Figure 34-7), an enzyme of pyrimidine nucleotide synthesis, salvages orotic acid by converting it to orotidine monophosphate (OMP). 

Many yeasts are inhibited by 5-fluorocytosine and a block in the synthesis of 5-fluorouridylic acid by loss of cytosine deaminase or of nracil phosphoribosyltransferase is sufficient to cause resistance. Mntational loss of pyrimidine salvage enzymes has been frequently observed. 

Pyrimidine Synthesis Pyrimidine Salvage Pyrimidine Degradation... 

As with purines, there is indirect evidence from studies in vitro that regenerating tetrathyridia of M. corti can synthesise pyrimidines de novo (315). Furthermore, aspartate transcarbamylase, the first enzyme in the pathway, has been demonstrated in Moniezia benedini (39), while five of the six pathway enzymes have been measured in H. diminuta (326). It appears, therefore, that at least some cestodes have the capacity to synthesise pyrimidines by the biosynthetic route. Little is known of pyrimidine salvage pathways in cestodes, although the key enzyme thymidine kinase has been... 

Figure 13.11. Structure of 5-fluorouracil (a), its activation by pyrimidine salvage enzymes (b), and its tautomerism that promotes ambiguous base pairing (c). While 5-FU is not very efficiently incorporated into DNA, the same mechanism applies to 5-bromonracil, whichbecanse of its closer steric resemblance of thymidine is readily incorporated.

Keywords Trichinella spiralis Trichinella pseudospiralis Thymidylate synthase Dihydrofolate reductase, dUTPase Pyrimidine salvage... 

The pyrimidine salvage pathway, which uses preformed pyrimidine bases from dietary sources or from nucleotide turnover, is of minor importance in... 

HAT Selection - The compounds hypoxanthine, aminopterin (see here), and thymidine (H,A, and T, respectively) can be used to select for cells having functional salvage pathways. Aminopterin inhibits dihydrofolate reductase, which blocks de novo purine and thymidine synthesis. Only cells which can utilize thymidine (pyrimidine salvage) and hypoxanthine (purine salvage) can grow in this medium. 

FIG. 6.13 Mammalian pyrimidine salvage and interconversion pathways. Enzymes listed in Figs 6.13-6.17 are as follows (1) deoxyCMP deaminase (2) thymidylate synthase (3) ribonucleotide reductase (4) deoxyuridine triphosphatase (5) CTP synthetase (6) nucleotide kinase (7) deoxyTMP kinase (8) nucleotide diphosphokinase (9) non-specific phosphatase or nucleotidase (10) cytidine kinase (11) pyrimidine phos-phorylase or hydrolase (12) uracil PRTase (13) cytidine deaminase (14) thymidine kinase (15) cytidine phosphotransferase (16) uridine phosphotransferase (17) thymidine phosphotransferase (18) deoxyribo-nucleotide phosphotransferase (19) cytosine PRTase. 

Consistent with the metabolic data, there is no dihydrofolate reductase/thymidylate synthase activity (75). Thymidine is salvaged by a phosphotransferase. Uracil PRTase, uridine phosphorylase, cytidine deaminase and uridine and cytidine phosphotransferases were found. The major pyrimidine salvaged is uracil via its PRTase. The lack of incorporation of salvaged uracil into DNA and the lack of thymidylate synthase indicates that this parasite cannot synthesize TMP from UMP. It is dependent on salvage for its thymidine requirements. This parasite possesses a hydroxyurea-resistant ribonucleotide reductase and can synthesize deoxycytidine nucleotides from cytidine nucleotides. 

FIG. 6.14 Thitrichomonas foetus pyrimidine salvage and interconversion pathways. For identity of enzymes see legend to Fig. 6.13. 

In summary, pyrimidine metabolism in the kinetoplastids is functionally similar to that found in mammalian cells. In the de novo synthesis of UMP, these parasites differ in two respects dihydro-orotate oxidase is cytoplasmic and not mitochondrial and the last two enzymes of UMP synthesis are glycosomal instead of cytoplasmic. Pyrimidine salvage by these parasites is more diverse than that of mammalian cells but, unlike the purines, de novo synthesis plays the major role. 

Eimeria tenella. It appears that the sporulated oocysts of E. tenella are capable of de novo pyrimidine synthesis since sporozoites incorporate " C-labeled aspartate and orotate into pyrimidine nucleotides (103). Besides de novo synthesis, E. tenella is also capable of pyrimidine salvage. E. tenella will incorporate uracil, cytidine and uridine but not thymidine into its nucleic acids (104). This parasite is dependent on UMP for the synthesis of TMP and thymidylate synthase activity is present in extracts of E. tenella (105). As with both Plasmodium and the kinetoplastids, the thymidylate synthase of E. tenella exists as a bifunctional protein (91). 

Evidence for pyrimidine de novo biosynthesis has also been found in other trematodes. Its enzymes are present in Fasciola gigantica (81), Paragonimus ohirai (112) and Clonorchis sinensis (112). De novo biosynthesis was verified by demonstrating the incorporation of [ C]bicarbonate into the C-2 position of uracil in P. ohirai (112). The pyrimidine salvage enzymes, uridine kinase and thymidine kinase, are found in P. ohirai and C. sinensis extracts (112). Aspartate transcarbamoylase is found in Fasciola hepatica and Paramphistomum cervi extracts (113). 

There is good evidence for de novo pyrimidine synthesis in cestodes. Five of the six enzymes needed for UMP synthesis are present in Hymenolepis diminuta and aspartate transcarbamoylase activity has been found in Moniezia benedeni (81). Salvage of preformed pyrimidines by a cestode was first reported in Mesocestoides corti (70). Thymidine kinase is the only cestode H. diminuta) pyrimidine salvage enzyme that has been characterized (114). 

A primary site of regulation is the synthesis of PRPP. PRPP synthetase is negatively affected by GDP and, at a distinct allosteric site, by ADR Thus, the simultaneous binding of an oxypurine (eg., GDP) and an aminopurine (eg., ADP) can occur with the result being a synergistic inhibition of the enzyme. This enzyme is not the committed step of purine biosynthesis PRPP is also used in pyrimidine synthesis and both the purine and pyrimidine salvage pathways. 

S ATP + uridine <3-5> (<3> rate-limiting enzyme in anabolism of uridine and cytidine [10] <3> rate limiting enzyme of pyrimidine salvage pathway [10] <4,5> first enzyme of pyrimidine salvage pathway [6] <3> enzyme is part of the anabolic pathway by which the preformed pyrimidine nucleosides are salvaged for nucleic acid biosynthesis [8]) (Reversibility <3-6> [6, 8, 10]) [6, 8, 10]... 

Figure 5. Pyrimidine salvage and nucleotide synthesis in Leishmania. Abbreviations are described in the text. Cray text box and arrow indicate that UK enzymatic activity has not been detected. The broken arrow represents the enzymatic steps for conversion of UMP intoall other pyrimidine nucleotides as described in the text.

The presence of a de novo synthesis pathway implies that pyrimidine sahmge may be functionally redundant and potentially less critical to the nutrition of the parasite than purine acquisition. Similar to purine salvage, pyrimidine salvage is initiated by the translocation of pyrimidine nucleosides or nucleobases across the parasite cell membrane via specific transporters. As described earlier, leishmanial NTl is responsible for the transport of pyrimidine nucleosides. The sole pyrimidine nucleobase transport activity in Leidrmania has been dtantctetized biochemically in L. major (LmUl) and shown to rect nize uracil exdusively and with hi affinity, but has not yet been doned. 

A common strata for drug devdopment is to target enzymes essential for viability. While it is unlikely that pyrimidine salvage is essential, the possibility that de novo pyrimidine biosynthesis is necessary for growth and virulence of L. donovani was tested by creating a Acps knodcout via... 

LaFon SW, Nelson DJ, Berens RL et al. Purine and pyrimidine salvage pathways in Leishmania donovani. Biochem Pharmacol 1982 31(2) 231-238. 

The salvage pathway utilizes preformed pyrimidines and purines for the synthesis of nucleic acids and is highly active in various types of cells. Uridine kinase plays a key role in the pyrimidine salvage pathway and its concentration is considered to reflect the relative efficiency of the system in utilizing preformed pyrimidines [74]. Adenosine kinase plays a similar role in making use of preformed purines [75]. It should be noted, however, that uridine and adenosine kinases are not the only enzymes involved in the salvage pathway and other deoxynucleoside kinases, phosphorylases, and phosphoribosyltransferases [76] also have important roles. 

One of the interesting features of dCD is that its catalytic activity depends on the feedback regulation based on the ratio of dCTP to dTTP, which are both the end products of the pyrimidine salvage pathway, wherein dCTP functions as an activator and dTTP, as an inhibitor. Both dCTP and dTTP appear to... 

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