Purines de novo synthesis

Folic acid antagonist inhibits dihydrofolate reductase (DHFR) blocks reduction of folate to tetrahydrofolate inhibits de novo purine synthesis results in arrest of DNA, RNA, and protein synthesis... 

Orotic acid in the diet (usually at a concentration of 1 per cent) can induce a deficiency of adenine and pyridine nucleotides in rat liver (but not in mouse or chick liver). The consequence is to inhibit secretion of lipoprotein into the blood, followed by the depression of plasma lipids, then in the accumulation of triglycerides and cholesterol in the liver (fatty liver) [141 — 161], This effect is not prevented by folic acid, vitamin B12, choline, methionine or inositol [141, 144], but can be prevented or rapidly reversed by the addition of a small amount of adenine to the diets [146, 147, 149, 152, 162]. The action of orotic acid can also be inhibited by calcium lactate in combination with lactose [163]. It was originally believed that the adenine deficiency produced by orotic acid was caused by an inhibition of the reaction of PRPP with glutamine in the de novo purine synthesis, since large amounts of PRPP are utilized for the conversion of orotic acid to uridine-5 -phosphate. However, incorporation studies of glycine-1- C in livers of orotic acid-fed rats revealed that the inhibition is caused rather by a depletion of the PRPP available for reaction with glutamine than by an effect on the condensation itself [160]. 

Decreased levels of folate coenzymes needed for various reactions of de novo purine synthesis and thymine synthesis produce shortages of deoxyribonudeotides and consequent impaired DNA synthesis in many tissues. 

Mycophenolate mofetil (MMF, CellCept) is an ester prodrug of mycophenolic acid (MPA), a Penicillium-de-rived immunosuppressive agent (see Chapter 57) that blocks de novo purine synthesis by noncompetitively inhibiting the enzyme inosine monophosphate dehydrogenase. MPA preferentially suppresses the proliferation of cells, such as T and B lymphocytes, that lack the purine salvage pathway and must synthesize de novo... 

Thioguanine is a purine analogue structurally related to 6-mercaptopurine and azathioprine. Thioguanine interferes with several enzymes required for de novo purine synthesis, and its metabolites are incorporated into DNA and RNA, further impeding nucleic acid synthesis. The mechanism of action of thioguanine in psoriasis is not clearly understood it has been hypothesized to affect the proliferation and trafficking of lymphocytes as well as the proliferation of keratinocytes. 

Mycophenolate mofetil (CdlCept), in conjunction with cyclosporine and corticosteroids, has clinical applications in the prevention of organ rejection in patients receiving allogeneic renal and cardiac transplants. By effectively inhibiting de novo purine synthesis, it can impair the proliferation of both T and B lymphocytes. Following oral administration, mycophenolate mofetil is almost completely absorbed from the GI tract, metabolized in the liver first to the active compound my-cophenolic acid, and then further metabolized to an inactive glucuronide. 

When interpreting these results, it should be taken into consideration that RBC 6-TGN levels can act only as a surrogate marker for the situation in the leukemic blast and may not ideally reflect incorporation of 6-TGN into the DNA and RNA of target cells. This process is likely to be influenced by other thiopurine metabolites such as 6-MMPR and their effects on incorporation and cytotoxicity through inhibition of de novo purine synthesis (101,103). 

Dervieux T, Brenner TL, Hon YY et al. De novo purine synthesis inhibition and antileukemic effects of mercaptopurine alone or in combination with methotrexate in vivo. Blood2002 100 1240-1247. 

PRPP is an "activated pentose" that participates in the synthesis of purines and pyrimidines, and in the salvage of purine bases (see p. 294). Synthesis of PRPP from ATP and ribose 5-phosphate is catalyzed by PRPP synthetase (ribose phosphate pyrophosphokinase, Figure 22.6). This enzyme is activated by inorganic phosphate (Pi) and inhibited by purine nucleotides (end-product inhibition). [Note The sugar moiety of PRPP is ribose, and therefore ribonucleotides are the end products of de novo purine synthesis. When deoxy-ribonucleotides are required for DNA synthesis, the ribose sugar moiety is reduced (see p. 295).]... 

The enzyme deficiency results in increased levels of PRPP and decreased IMP and GMP, causing increased de novo purine synthesis. 

Azathioprine inhibits purine synthesis, which is necessary for the proliferation of cells, especially immunocompetent cells. It is converted to 6-mercaptopurine after it reacts with glutathione, and its metabolite, 6-mercaptopurine, is converted to additional metabolites, which inhibit de novo purine synthesis. This results from the synthesis of 6-thio IMP, 6-thio GMP and 6-thio GTP, and cell proliferation is inhibited after 6-thio GTP is inserted into host DNA. 

Kan,J. L.jJannatipour, M., Taylor, S. M., and Moran, R. G. (1993). Mouse cDNAs encoding a trifunctional protein of de novo purine synthesis and a related single-domain glyci-namide ribonucleotide synthetase. Gene, 137, 195—202. 

Which of the following antitumor agents work by impairing de novo purine synthesis ... 

Overproduction of uric acid can occur due to excessive de novo purine synthesis, excessive dietary purines, or the conversion of tissue nucleic acid to purine nucleotides. When these purines are metabolized, the by-products are converted to uric acid by the enzyme xanthine oxidase. Increased levels of uric acid result if the overproduction exceeds excretion. Underexcretion of uric acid can be due to defects in the renal tubular mechanisms that regulate uric acid levels in the body, causing decreased filtration, decreased secretion, or increased reabsorption. 

Aathioprine/6-MP are catalyzed to inhibitors of enzymes that are important in de novo purine synthesis. 

On the other hand, many of the purine nucleoside analogues act as feedback inhibitors of de novo purine synthesis, but only after conversion to their 5 -phosphates. The reaction under regulatory control is the first committed... 

Although many compounds pharmacologically able to inhibit de novo purine synthesis are known, none has been useful in the treatment of overproduction of uric acid because of their general toxicity, since they are not specific for the overproduction per se. 

Two diastereomers of 5,10-dideaza-5,6,7,8-tetrahydrofolic acid, DDATHF, 127, both potent inhibitors of folate metabolism and de novo purine synthesis , have been synthesized by catalytic reduction of the unsaturated intermediate diethyl 2-acetyl-5,10-dideaza-9,10-didehydrofolate with Adams catalyst and carrier-free tritium gas in AcOH and H20 solution. Each of the separated (6R) and (6S) diastereomers had specific activity 11.2 Ci mmol and contained tritium almost exclusively at the metabolically stable positions Cjj, C, C J, C gp C o, and the phenyl ring of DDATHF. 

Azathioprine (3 to 10 mg/kg) prevents transplant rejection. It is reserved for patients deemed unresponsive to cyclosporine and prednisone. Azathioprine is cleared to 6-mercaptopurine, which in turn can be converted to 6-mercaptopurine nucleotides leading to an inhibition of de novo purine synthesis or anabolism to thio-IMP, which, as a fraudulent nucleotide, can interfere with the salvage pathway of purine synthesis. Thio-IMP is subsequently converted to thio-GMP and eventually thio-GTP, leading to DNA damage upon intercalation of thio-GMP into the DNA backbone (see Figure 28). Azathioprine is a more effective immunosuppressant than is mercaptopurine (Purinethol). 

Thioguanine has multiple metabolic effects. Its tumor inhibitory properties may be due to one or more of its effects on feedback inhibition of de novo purine synthesis inhibition of purine nucleotide interconversions or incorporation into DNA and RNA. The net conseqnence of its actions is a sequential blockade of the synthesis and utilization of the purine nucleotides. 

Figure 22.10 illustrates the pathway for de novo pyrimidine biosynthesis. It differs from de novo purine synthesis in that the pyrimidine ring is synthesized separate from the ribose sugar (in purines, the ring is built upon the sugar - see Figure 22.4). In addition, de novo pyrimidine biosynthesis is not branched. Synthesis leads to UMP, from which CTP is ultimately made. By contrast, de novo purine biosynthesis branches after IMP is produced (Figure 22.6).

Purine salvage enzymes - The drugs allopurinol (see here) and formycin B inhibit the action of cellular purine salvage enzymes. Thus, these drugs can be used to treat individuals infected by the parasitic protozans, Plasmodium, and Leishmania because these parasites lack the capacity for de novo purine synthesis (i.e., they depend entirely upon cellular purine salvage enzymes and bases provided by the host)... 

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