The major site of purine synthesis is in the liver
Nucleotide - Wikipedia
Purine and Pyrimidine Metabolism - Eccles Health …
Because nearly all cells possess de novo nucleotide synthetic capabilities, the salvage enzymes are usually not required for cell viability. In addition, a large number of nucleobase and nucleoside antimetabolites are available, most of them inhibitors of specific enzymes (see Nucleotides, Nucleosides, And Nucleobases). These factors create favorable conditions for the use of salvage enzymes as selectable genetic markers, ie, genetic characteristics that promote the selective survival or growth of desired cell types. For example, the mammalian enzyme encoding HGPRT is widely used as a selectable marker in genetic analysis. 6-Thioguanine is metabolized by HGPRT to give the thiol analogue of inosinic acid, which is toxic. Cells lacking HGPRT can be selected for because they grow in 6-thioguanine-containing medium, while other cells are killed. Hence, one can estimate mutation rates by culturing wild-type cells in the presence of thioguanine and enumerating the cells that grow. Another advantage of this gene for genetic analysis is that it is carried on the X-chromosome. Thus, in male-derived cell lines, only one mutation, rather than two independent events is required to give the resistant phenotype.
On the other hand, one can select for the presence of active salvage enzymes. The best example is the use of "HAT medium" in somatic cell genetic analysis and in preparing monoclonal antibodies. These techniques involve fusing cells of different origins and culturing in HAT medium to select for those cells that have undergone fusion. HAT is an acronym for the medium’s constituents,— hypoxanthine, aminopterin, and thymidine. Aminopterin inhibits dihydrofolate reductase, blocking the synthesis of tetrahydrofolate needed for de novo synthesis of purine nucleotides and thymidine nucleotides. Thus, cells can grow in HAT medium only if they express active thymidine kinase and HGPRT, for salvage synthesis of thymidine and purine nucleotides, respectively. In monoclonal antibody production, one of the cell lines to be fused lacks thymidine kinase, and the other lacks HGPRT. Thus, only cells resulting from a fusion event have functional copies of both enzymes and can grow.
Amino acid synthesis - Wikipedia
A final example relates to the use of a selectable marker to force expression of a nonselected marker. For cloning into expression systems in mammalian cells, one often incorporates into the cloning vector the Escherichia coli xpt gene, which encodes a distinctive phosphoribosyltransferase that acts on xanthine and guanine. During and after transformation of cells with the recombinant DNA, the cells are cultured in the presence of mycophenolic acid, which blocks de novo guanine nucleotide synthesis by inhibiting IMP dehydrogenase, the enzyme that converts IMP to XMP (which would then be converted to GMP). Thus, the only cells that can grow are those that have taken up and expressed the xpt gene, which bypasses this metabolic block. Being carried on the same vector, the gene of interest is also cloned and/or expressed, even though its expression was not directly selected for.
As an example of the factors involved, consider the fluorinated pyrimidines, 5-fluorouracil (FUra) and 5-fluorodeoxyuridine (FdUrd), analogues used for four decades in treating various cancers. It was established in 1958 (9) that these drugs are converted in vivo to 5-fluorodeoxyuridine monophosphate (FdUMP), an analogue of deoxyuridine monophosphate, the substrate for thymidylate synthase (Fig. 1; see Deoxyribonucleotide Biosynthesis And Degradation), and that FdUMP is a potent inhibitor of thymidylate synthase and, hence of DNA replication. Figure 1 shows also the metabolic pathways that both activate these analogues and divert them from their desired endpoint (10). From the figure, one can see that coadministration with FdUrd of a thymidine phosphorylase inhibitor should increase the effectiveness of the analogue by blocking its catabolism. Note that there are multiple routes for activation of FUra; note also that FUra can enter pools of RNA precursors which, in principle, could limit its selectivity by diminishing the specificity of its effect against DNA synthesis. There is evidence, however, that, in some tumors, the effectiveness of FUra actually depends in part on its incorporation into RNA, where it stimulates translational miscoding.
Multiple Choice Questions- Molecular Biology Set-2 …
Purine and Pyrimidine Metabolism - EHSL
Chapter 27 - Columbia University
Molecular Biology | Biochemistry for Medics – Lecture …
Biochemistry: Purines and Pyrimidines
a) DNA Pol I
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