Synthesis of Purines

Because of the ready availability of nucleosides from natural sources, a frequently used route to substituted purines is via the manipulation of one of these. 

There are two general approaches to the construction of the purine ring system. Additionally, a category which can be defined as one pot methods, are adaptations of the type of process that probably took place in prebiotic times, when simple molecules, such as hydrogen cyanide and ammonia, are believed to have combined to give the first purines. 

5-Diamino-pyrimidines react with carboxylic acids, or derivatives, to give purines, the carboxyl carbon corresponding to C-8. 

Precursors to 9-substituted purines, therefore requiring a substituent on the pyrimidine-4-amino group, are available from the reaction of a 4-chloropyrimidine with a primary amine. 

When milder conditions are required for the cychsation, perhaps because of the presence of a sugar residue, an orthoester (often activated with acetic anhydride) or an acetal-ester (illustrated below) can be used. 

8-Unsubstituted purines can be prepared simply by heating 4,5-diaminopyrimidines with formic acid, but formamide (or formamidine ) are better. The reaction proceeds via cyclising dehydration of an intermediate formamide this usually takes place in situ using formamide but generally requires a second, more forcing step when formic acid is employed initially. Purine itself can be prepared by this route.  

Comparitively little information is available concerning any special reactivity associated with purine alkyl groups, but what is available suggests that their reactivity is comparable to pyridine a-alkyl substituents. 

Here again, comparatively little systematic information is available, but a parallel with pyridine a-acids can again be implied in that purine acids undergo decarboxylation on heating.  

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Glutamine as an important source for the synthesis of purines, pyrimidines and amino sugars is essential for most cell lines, too. High concentrations of glutamine may also effect cell growth indirectly as the major end-products are lactate and ammonia [15]. Both known to be toxic metabohtes. 

Davoll, J., Lythgoe, B., Todd, A.R. (1948) Experiments on the Synthesis of Purine Nucleosides. Part XIX. A Synthesis of Adenosine. Journal of the Chemical Society, 967-969. 

RNA World hypothesis The early synthesis of purines and the observation of ribozyme autocatalysis lead to the idea of RNA preceding DNA... 

The purines from which uric acid is produced originate from three sources dietary purine, conversion of tissue nucleic acid to purine nucleotides, and de novo synthesis of purine bases. 

The chemistry, metabolism, and clinical importance of folic acid have been the subject of many excellent reviews (A7, Gil, H14, H20, Rl). Folic acid deficiency leads to a macrocytic anemia and leucopenia. These symptoms are due to inadequate synthesis of nucleic acid. The synthesis of purine bases and of thymine, required for nucleic acid synthesis, is impaired in folic acid deficiency. Detection of folic acid activity in biologic fluids and tissues is of the utmost importance it distinguishes between the various anemias, e.g., those due to vitamin Bi2 or folic acid deficiency. Because morphology of the abnormal red cell does not help in diagnosing vitamin deficiency, one must rely on assay methods for differential diagnosis. Treatment of pernicious anemia with folic acid has led to subacute combined degeneration of the spinal cord despite... 

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. 

Donald Woods discovered that sulphonamides exerted their action by inhibiting an enzyme used by bacteria to synthesise folic acid. The compound 4-aminobenzoic acid is the precursor for folic acid, and is structurally similar to sulphonamide. Bacteria that were unable to synthesise folic acid were unable to achieve de novo synthesis of purines for their DNA and RNA synthesis and hence could not proliferate. Such competitive inhibitors, which mimicked normal metabolites, became known as antimetabolites (many are used in cancer chemotherapy. Chapter 21). 

Tumour cells also require glutamine as a fuel for energy generation and as a precursor for the synthesis of purine and pyrimidine nucleotides for DNA and RNA synthesis. The roles and importance of glutamine in tumour cells and possible competition between the cells for glutamine are discussed in Chapter 21. The pathway for the metabolism of glutamine is similar to that in the immune cells. 

Small methyl groups are important in the stractnre of some small compounds, nucleotides, some bases in DNA mole-cnles and in postranslational modification of amino acids in proteins. The transfer of a single carbon atom is important in the synthesis of purine nncleotides. The componnds involved in the whole process of methyl gronp transfer, and are carbon metolism, are methionine, homocysteine, serine and the vitamins, folic acid and B12. 

Figure 20.8 Summary of pathways for de novo synthesis of purine and pyrimidine nucleotides. C represents transfer of a single carbon atom (a one-carbon transfer). Details are provided in Appendix 20.1. IMP - inosine monophosphate. For thymi-dylate synthesis, see Figure 20.12a.

Methotrexate is an antimetabolite of folic acid and has immunosuppressant properties. It inhibits the enzyme dihydrofolate reductase that is required for the synthesis of purines and pyrimidines. It is used in malignant disease, Crohn s disease, rheumatic disease and psoriasis. Folic acid is given with methotrexate to reduce the occurrence of side-effects particularly the risk of mucositis. 

Tetrahydrofolic acid (THF) is a co-en-zyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and required for cell growth and repUcation. Lack of THF leads to inhibition of cell proUferation. 

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. 

The synthesis of purine nucleotides (1) starts from IMP. The base it contains, hypoxanthine, is converted in two steps each into adenine or guanine. The nucleoside monophosphates AMP and CMP that are formed are then phos-phorylated by nucleoside phosphate kinases to yield the diphosphates ADP and GDP, and these are finally phosphorylated into the triphosphates ATP and CTP. The nucleoside triphosphates serve as components for RNA, or function as coenzymes (see p. 106). Conversion of the ribonucleotides into deoxyribo-nucleotides occurs at the level of the diphosphates and is catalyzed by nucleoside diphosphate reductase (B). 

Nitrogen donator in synthesis of purine bases, cytosine, GIcNAc, His, and Trp... 

These compounds inhibit synthesis of purine nucleotides, which are made up of purine bases and phosphorylated ribose. Both compounds must be transformed into nucleotides by adding a phosphoribosyl fragment. 

If photosynthetic and respiratory changes cannot account for the increases in adenylate concentration, which system is responsible It has been reported that ADP and ATP concentrations of Ehrlich ascites tirnior cells increase in the presence of adenine (15), Whether this wo ild hold true for plant cells is not known, but it seems plausible that equilibrium shifts would initiate similar responses. An increase in adenine concentrations could occur if there was any breakdown of nucleic acids. There is one report that the number of ribosomes in the chloroplast does decrease in response to ozone (16). An increase in synthesis of purines is also possible but there is no evidence to either support or refute this hypothesis. 

Fig. 1. Folate-cobalamin interaction in the synthesis of purines and pyrimidines and, therefore, of DNA. (1) In gastrointestinal mucosa cells (2) in the liver (3) in peripheral tissues. C, cobalamine DAC, desoxyadenosylcobalamine HC, hydroxy cobalamine MC, methylcobalamine F, folic acid MTHF, methyltetrahydrofolic acid THF, tetrahydrofolic acid DHF, dihydrofolic acid dUMP, deoxyuridinemonophosphate dTMP, deoxythymidine-monophosphate. (Adapted from Far-...

Trimethoprim is a competitive inhibitor of the enzyme dihydrofolate reductase and can thus prevent the formation of tetrahydrofolate thereby blocking the synthesis of purines. The affinity of trimethoprim for the enzyme in microorganisms is 10,000 times higher than for the human enzyme which explains the selective toxicity. Used alone its main indication is acute uncomplicated urinary tract infections. It is then as effective as co-trimoxazole but has the advantage of fewer adverse reactions. 

Methotrexate is a folic acid analogue. Its mechanism of action is based on the inhibition of dihydrofolate reductase. Inhibition of dihydrofolate reductase leads to depletion of the tetrahydrofolate cofactors that are required for the synthesis of purines and thymidylate (see Fig. 2). Enzymes that are required for purine and thymidylate synthesis are also directly inhibited by the polyglutamates of methotrexate which accumulate with dihydrofolate reductase inhibition. The mechanisms that can cause resistance include decreased transport of methotrexate into the tumor cells, a decreased affinity of the antifolate for dihydrofolate reductase, increased concentrations of intracellular dihydrofolate reductase and decreased thymidylate synthetase activity. 

Mycophenolate mofetil is used together with cyclosporine and corticosteroids for the prophylaxis of acute organ rejection in patients undergoing allogeneic renal, or hepatic transplants. Compared with azathioprine it is more lymphocyte-specific and is associated with less bone marrow suppression, fewer opportunistic infections and lower incidence of acute rejection. More recently, the salt mycophenolate sodium has also been introduced. Mycophenolate mofetil is rapidly hydrolyzed to mycopheno-lic acid, its active metabolite. Mycophenolic acid is a reversible noncompetitive inhibitor of inosine monophosphate dehydrogenase, an important enzyme for the de novo synthesis of purines. As lymphocytes have little or no salvage pathway for purine... 

CS117 Fujimori, N. and H. Ashihara. CS128 Adenine metabolism and the synthesis of purine alkaloids in flowers of Camellia. Phytochemistry 1990 29(11) CS129... 

I I 3. The answer is c. (Hardman, pp 1243-1247.) Antimetabolites of folic acid such as methotrexate, which is an important cancer chemotherapeutic agent, exert their effect by inhibiting the catalytic activity of the enzyme dihydrofolate reductase. The enzyme functions to keep folic acid in a reduced state. The first step in the reaction is the reduction of folic acid to 7,8-dihydrofolic acid (FH2), which requires the cofactor nicotinamide adenine dinucleotide phosphate (NADPH). The second step is the conversion of FH2 to 5,6,7,8-tetrahydrofolic acid (FH ). This part of the reduction reaction requires nicotinamide adenine dinucleotide (NADH) or NADPH. The reduced forms of folic acid are involved in one-carbon transfer reactions that are required during the synthesis of purines and pyrimidine thymidylate. The affinity of methotrexate for dihydrofolate reductase is much greater than for the substrates of folic acid and FH2. The action of... 

Or6, J. and Kimball, A. P. (1961). Synthesis of purines under possible primitive Earth conditions. 1. Adenine from hydrogen cyanide. Arch. Biochem. Biophys., 94,... 

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