Pyrimidine nucleotides,inhibition

Low activities of orotidine phosphate decarboxylase and (usually) orotate phosphoribosyltransferase are associated with a genetic disease in children that is characterized by abnormal growth, megaloblastic anemia, and the excretion of large amounts of orotate. When affected children are fed a pyrimidine nucleoside, usually uridine, the anemia decreases and the excretion of orotate diminishes. A likely explanation for the improvement is that the ingested uridine is phosphorylated to UMP, which is then converted to other pyrimidine nucleotides so that nucleic acid and protein synthesis can resume. In addition, the increased intracellular concentrations of pyrimidine nucleotides inhibit carbamoyl phosphate synthase, the first enzyme in the. naibwav of aro-tate synthesis. 

Nucleotide levels apparently regulate CPS activity (O Neal and Naylor, 1968,1976 Ong and Jackson, 1972a,b Parker and Jackson, 1981). Pyrimidine nucleotides inhibit CPS while purine nucleotides (GMP, IMP) stimulate activity. Ornithine activated CPS and could overcome inhibition by pyrimidines whereas arginine did not affect the activity of CPS from any of these sources. Metabolic regulation of plant OCT has not been reported. Compartmentaliza-tion at the subcellular and tissue levels as well as developmental control are certainly important in the regulation of this key pathway. Molecular, biochemical, and immunological approaches will all be necessary to unravel the regulatory features of these key enzymes and metabolic pathways. 

Purine and pyrimidine biosynthesis parallel one another mole for mole, suggesting coordinated control of their biosynthesis. Several sites of cross-regulation characterize purine and pyrimidine nucleotide biosynthesis. The PRPP synthase reaction (reaction 1, Figure 34-2), which forms a precursor essential for both processes, is feedback-inhibited by both purine and pyrimidine nucleotides. 

The increase in erythrocyte destruction may be due in part to inhibition by lead of pyrimidine-5 -nucleotidase, which results in an accumulation of pyrimidine nucleotides (cytidine and uridine phosphates) in the erythrocyte or reticulocyte. This enzyme inhibition and nucleotide accumulation affect erythrocyte membrane stability and survival by alteration of cellular energetic (Angle et al. 1982 EPA 1986a). Formation of the heme-containing cytochromes is inhibited in animals treated intraperitoneally or orally... 

Not all analogues become active against cancer cells through incorporation into nucleic acid. Some analogues block the synthesis of normal purine and pyrimidine nucleotides for example, 8-azaguanine blocks guanosine monophosphate (GMP) synthesis and 6-mercaptopurine inhibits adenosine monophosphate (AMP) syn-thesis. 

Reisner M, Carmeli S, Werman M, Sukenik A (2004) The cyanobacterial toxin cylindrospermopsin inhibits pyrimidine nucleotide synthesis and alters cholesterol distribution in mice. 

Since one of the catalytic actions of reverse transcriptase is DNA synthesis, analogues of pyrimidine and purine nucleotides inhibit this process. A drug, zidovudine, azi-... 

Antimetabolites (inhibition of purine and pyrimidine nucleotide synthesis) Methotrexate Folic acid antagonist, inhibits tetrahydrofolate reductase and therefore dTMP synthesis 6-Mercaptopurine Interferes with purine synthesis 5-Fluorouracil Inhibits dTMP synthesis ... 

The fact that many agents which interrupt the synthesis of pyrimidine nucleotides from orotic acid in animals can also inhibit the growth of experimental neoplasms suggests a search for additional antimetabolites whose locus of action is in this metabolic sequence. Two in vitro biological screening systems were developed for this purpose [202—207]. From a study of systems with oxidative energy sources, 5-bromo-[208—209] (Villa), 5-chloro-[210] (Vlllb) and 5-diazo-orotic acid [211] (IX) were found to inhibit the conversion of orotic acid to the uridine nucleotides by 40—100 per cent [202]. 

Pemetrexed is chemically similar to folic acid. It inhibits three enzymes used in purine and pyrimidine synthesis - thymidylate synthetase, dihydrofolate reductase, and glycinamide ribonucleotide formyl transferase. By inhibiting the formation of precursor purine and pyrimidine nucleotides, pemetrexed prevents the formation of DNA and RNA. In 2004 it was approved for treatment of malignant pleural mesothelioma and as a second-line agent for the treatment of non-small cell lung cancer. Adverse effects include gastrointestinal complaints, bone marrow suppression, alopecia, allergic and neurotoxic reactions. 

The pyrimidine antagonists inhibit the biosynthesis of pyrimidine nucleotides or interfere with vital cellular functions, such as the synthesis or function of nucleic acids. The analogues of deoxycytidine and thymidine that are used are inhibitors of DNA synthesis while 5-fluorouracil (5-FU) an analogue of uracil, is an inhibitor of both RNA function and of the synthesis of thymidylate (see Fig. 2). PALA (N-phosphonoacetyl-L-aspartate), an inhibitor of as-... 

Pyrimidine Nucleotide Biosynthesis Is Regulated by Feedback Inhibition... 

Regulation of the rate of pyrimidine nucleotide synthesis in bacteria occurs in large part through aspartate transcarbamoylase (ATCase), which catalyzes the first reaction in the sequence and is inhibited by CTP, the end product of the sequence (Fig. 22-36). The bacterial ATCase molecule consists of six catalytic subunits and six regulatory subunits (see Fig. 6-27). The catalytic subunits bind the substrate molecules, and the allosteric subunits bind the allosteric inhibitor, CTP. The entire ATCase molecule, as well as its subunits, exists in two conformations, active and inactive. When CTP is... 

The sugar configuration about the T, 2, 3, and 4 positions can be changed by synthesis. A variety of pyrimidine nucleoside cyclic phosphates have been made. Ukita et al. (425) prepared -D-lyxo-uridine 2 3 -cyclic phosphate (Fig. 21a). The configuration about the 2 and 3 positions is inverted and the two OH groups are now cis to the base rather than trans as in the D-ribose series. No hydrolysis at all of this compound was observed in the presence of RNase. However, both the cyclic phosphate and the free 2 (3 )-nucleotides inhibit the enzyme. The... 

Acivicin is a potent inhibitor of several steps in purine nucleotide biosynthesis that utilize glutamine. The enzymes it inhibits are glutamine PRPP amidotransferase (step 1, fig. 23.10), phosphoribosyl-A-formylglycinamidine synthase (step 4, fig. 23.10), and GMP synthase (see fig. 23.11). In pyrimidine nucleotide biosynthesis the enzymes inhibited are carbamoyl synthase (step 1, fig. 23.13) and CTP synthase (see fig. 23.14). Acivicin is under trial for the treatment of some forms of cancer. 

In eukaryotes, carbamoyl phosphate synthase is inhibited by pyrimidine nucleotides and stimulated by purine nucleotides it appears to be the most important site of feedback inhibition of pyrimidine nucleotide biosynthesis in mammalian tissues. It has been suggested that under some conditions, orotate phosphoribosyltransferase may be a regulatory site as well. 

Carbamyl-L-aspartate is the key precursor in the biosynthesis of pyrimidines. The enzyme aspartate transcarbamylase is inhibited by several pyrimidine nucleotides, notably cytidine triphosphate, and is activated by ATP, a purine nucleotide. Thus the enzyme is under feedback regulation, and controls the relative concentration of pyrimidine and purine nucleotides. 

Fluorouracil and fluorodeoxyuridine (floxuridine) inhibit pyrimidine nucleotide biosynthesis and interfere with the synthesis and actions of nucleic acids. To exert its effect, fluorouracil (5-FU)... 

Fluorouracil and fluorodeoxyuridine (floxuridine) inhibit pyrimidine nucleotide biosynthesis and interfere with the synthesis and actions of nucleic acids. To exert its effect, fluorouracil (5-FU) must first be converted to nucleotide derivatives such as 5-fluorodeoxyuridylate (5-FdUMP). Similarly, floxuridine (FUdR) is also converted to FdUMP by the following reactions ... 

Eukaryotic organisms contain a multifunctional enzyme with carbamoylphosphate synthetase, aspartate transcarbamoylase, and dihydroorotase activities. Two mechanisms control this enzyme. First, control at the level of enzyme synthesis exists the transcription of the gene for the enzyme is reduced if an excess of pyrimidines is present. Secondly, control exists at the level of feedback inhibition by pyrimidine nucleotides. This enzyme is also an example of the phenomenon of metabolic channeling aspartate, ammonia, and carbon dioxide enter the enzyme and come out as orotic acid. 

Carbamoyl phosphate synthetase is a rate-controlling step in the pyrimidine biosynthetic mechanism. It is inhibited by UTP and other pyrimidine nucleotides. 

A large number of compounds in category 3 act at different sites in the pathways for purine and pyrimidine biosynthesis. These compounds are very toxic for rapidly growing tumors and bacteria, making them useful in cancer chemotherapy and treatment of bacterial infections. 6-Mercaptopurine is a potent inhibitor of purine biosynthesis, and 5-fluorouracil inhibits thymidylate synthesis. Some compounds, such as hydroxyurea and sulfonamides, inhibit the synthesis of both purine and pyrimidine nucleotides. These are only a few of the many compounds useful in treating cancer and infectious diseases (see Chapter 10). 

Antimetabolites are structurally related to normal cellular components. They generally interfere with the availability of normal purine or pyrimidine nucleotide precursors by inhibiting their synthesis or by competing with them in DNA or RNA synthesis. Their maximal cytotoxic effects S-phase (and therefore cell-cycle) specific. 

In this experiment we will examine some of the properties of the aspartate transcarbamylase of Escherichia coli, which is typical of many enzymes subject to feedback inhibition and which has been studied extensively. Aspartate transcarbamylase (ATCase) catalyzes the first reaction unique to the biosynthesis of pyrimidine nucleotides. ATCase is subject to specific inhibition by quite low concentrations of one of its end products, cytidine 5 -triphosphate (CTP). This relationship and two other regulatory interactions important to the control of pyrimidine biosynthesis are summarized in Figure 9-1. 

Two activities of the immunosuppressive metabolite of Leflimomide, A77 1 726. Inhibition of pyrimidine nucleotide synthesis and protein tyrosine phosphorylation. Biochem. Pharmacol. 52,... 

In contrast, some data suggest that P2Y receptor agonists can inhibit cytokine release from aetivated spinal cord microglia (Gerevich and Illes, 2004). This process could interrupt chronic pain development and continuation. Thus, UTP and UDP were shown to be analgesic in the neuropathic pain model (Okada et al., 2002). IT pyrimidine nucleotides elevated the nociceptive threshold in the paw pressure and... 

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