Pyrimidine salvage synthesis

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... 

While the PRTases salvage nucleobases within cells, nucleosides such as adenosine and uridine are present in the blood at much higher concentrations ( 1 pM) than the equivalent nucleobases, adenine and uracil. Indeed, the brain synthesizes pyrimidine nucleotides (UTP and CTP) via salvage synthesis from uridine produced by the liver and released into the circulation. Human cells may contain at least three types of nonspecific nucleoside transporters, and nucleosides are internalized more rapidly than nucleobases. 

Enzymes that act on PRPP include Phosphoribosyltransferases (salvage synthesis and de novo synthesis of pyrimidines), PRPP amidotransferase... 

See also De Novo Biosynthesis of Purine Nucleotides, De Novo Pyrimidine Nucleotide Metabolism, Nucleotide Salvage Synthesis... 

UMP is a nucleotide intermediate in pyrimidine biosynthesis. It is produced by salvage synthesis and by the de novo pathway shown in Figure 22.10. 

Pyrimidine Nucleotide Metabolism, Nucleotide Salvage Synthesis... 

See also The Importance of PRPP, De Novo Biosynthesis of Purine Nucleotides, Excessive Uric Acid in Purine Degradation, De Novo Pyrimidine Nucleotide Metabolism, Nucleotide Salvage Synthesis, Deoxyribonucleotide Biosynthesis, Biosynthesis of Thymine Deoxyribonucleotides, Salvage Routes to Deoxyribonucleotide Synthesis... 

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. 

Salvage Synthesis and Pyrimidine Catabolism (Figure 22.11) Deoxyribonucleotide Biosynthesis and Metabolism (Figure 22.12)... 

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). 

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. 

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. 

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. 

In many cells, the capacity for de novo synthesis to supply purines and pyrimidines is insufficient, and the salvage pathway is essential for adequate nucleotide synthesis. In patients with Lesch-Nyhan disease, an enzyme for purine salvage (hypoxanthine guanine phosphoribosyl pyrophosphate transferase, HPRT) is absent. People with this genetic deficiency have CNS deterioration, mental retardation, and spastic cerebral palsy associated with compulsive self-mutilation, Cells in the basal ganglia of the brain (fine motor control) normally have very high HPRT activity. These patients also all have hyperuricemia because purines cannot be salvaged. 

De novo synthesis of purines and pyrimidines yields the monophosphates IMP and UMP, respectively (see p. 188). All other nucleotides and deoxynucleotides are synthesized from these two precursors. An overview of the pathways involved is presented here further details are given on p. 417. Nucleotide synthesis by recycling of bases (the salvage pathway) is discussed on p. 186. 

A. Salvage pathways allow synthesis of nucleotides from free purines or pyrimidines that arise from nucleic acid degradation or dietary sources, which is more economical for the cell than de novo synthesis. 

Purine and pyrimidine nucleotides are essential for a vast number of biological processes such as the synthesis of RNA, DNA, phospholipids, glycogen, and the si-alylation and glycosylation of proteins. Both purines and pyrimidines can be synthesized de novo in mammalian cells through multistep processes. In addition to the de novo synthesis, purine nucleotides can also be synthesized via the salvage of... 

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