Formation of AMP and GMP

The rescue of normal, but not tumor, cells from methotrexate toxicity by folinic acid is partly explained by differences in membrane transport. For example, osteogenic sarcoma cells (which do not respond to conventional doses of methotrexate treatment) are not rescued by folinic acid administered after methotrexate, presumably owing to the absence of transport sites for folinic acid in the neoplastic cells. The therapeutic effects of administration of methotrexate and rescue with folinic acid are superior to those of methotrexate alone. Resistance to methotrexate can develop from increased activity of dihydrofolate reductase, synthesis of an enzyme having a lower affinity for the inhibitor, decreased transport of the drug into tumor cells, decreased degradation of the reductase, and genetic amplification of the gene for dihydrofolate reductase. 

Azaserine (L-serine diazoacetate), isolated from a species of Streptomyces, is a structural analogue of glutamine 

A variety of inhibitors of purine biosynthesis function at different stages and are used as antimicrobial, anticancer, and immunosuppressive agents. 

The sulfonamide drugs were the first effective antibacterial agents to be employed systemically in humans. These drugs resemble p-aminobenzoic acid in structure and inhibit utilization of that compound for the synthesis of folate in bacteria. Sulfonamides do not interfere with human metabolism. 

Mycophenolic acid and ribavarin monophosphate inhibit IMP dehydrogenase and hence GMP synthesis. 

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The base is directly ribosylated with PRPP by PT. The two enzymes concerned with purine salvage are adenine phosphoribosyl transferase (APRT), which catalyzes the formation of AMP, and hypoxanthine-guanine phosphoribosyl transferase (HGPRT), which catalyzes the formation of GMP as well as IMP, the precursor of GMP and AMP (Figure 6.58). Purine nucleotide phosphorylases (PNPases) can also... 

The biosynthesis of the purines is also regulated by feedback inhibition. ADP and GDP inhibit the formation of PRRPP from ri-bose-5 -phosphate. Similarly, step 2a is inhibited by AMP and GMP. 

Three major feedback mechanisms cooperate in regulating the overall rate of de novo purine nucleotide synthesis and the relative rates of formation of the two end products, adenylate and guanylate (Fig. 22-35). The first mechanism is exerted on the first reaction that is unique to purine synthesis—transfer of an amino group to PRPP to form 5-phosphoribosylamine. This reaction is catalyzed by the allosteric enzyme glutamine-PRPP amidotransferase, which is inhibited by the end products IMP, AMP, and GMP. AMP and GMP act synergisti-cally in this concerted inhibition. Thus, whenever either AMP or GMP accumulates to excess, the first step in its biosynthesis from PRPP is partially inhibited. 

Conversion of IMP to AMP and GMP. In both cases two steps are required. Note that the formation of AMP requires GTP, and the formation of GMP requires ATP. This tends to balance the flow of the IMP down the two pathways. 

In the first step of AMP synthesis, the C-6 keto oxygen of the hypoxanthine base moiety of IMP is replaced by the amino group of aspartate. In the second step the product of the first reaction, adenylosuccinate, is hydrolyzed to form AMP and fumarate. GMP synthesis begins with the oxidation of IMP to form XMP. GMP is produced as the amide nitrogen of glutamine replaces the C-2 keto oxygen of XMP. Note that AMP formation requires GTP and GMP formation requires ATP. 

Lotta Topaigne is being treated with allopurinol for gout, which is caused by an accumulation of sodium urate crystals in joints and joint fluid, particularly in the ankle and great toe. Allopurinol is a suicide inhibitor of the enzyme xanthine oxidase, which is involved in the degradation of purine nucleotides AMP and GMP to uric acid (urate). Although hypoxanthine levels increase in the presence of allopurinol, hypoxanthine does not participate in urate crystal formation and precipitation at this concentration. It is excreted in the urine. 

Both AMP and GMP inhibited purine synthesis at the level of formation of phosphoribosylamine irrespective of whether glutamine or ammonia was the N-donor. Detailed analysis of the AMP studies however was difficult because of the rapid enzymatic deamination of AMP with this enzyme preparation in the absence of GTP. 

In bone marrow extracts, IMP served as a precursor of both AMP and GMP (148-150). The transformation of IMP to AMP by the marrow enzyme system was dependent upon the addition of L-aspartic acid and a high energy phosphate source. Adenylosuccinic acid (Fig. 12), which was isolated and identified at the same time, was established as the intermediate in the formation of AMP from IMP. (13S, 136,151,158) The formation of adenylosuccinic acid [Eq. (15)] required GTP specifically and was studied extensively in E. coli (133) the enzyme catalyzing this reaction was named adenylosuccinate synthetase. 

Formation constants for complex species of mono-, di-, and trialkytin(rV) cations with some nucleotide-5 -monophosphates (AMP, LIMP, IMP, and GMP) are reported by De Stefano et al. The investigation was performed in the light of speciation of organometallic compounds in natural fluids (I = 0.16-1 moldm ). As expected, owing to the strong tendency of organotin(IV) cations to hydrolysis (as already was pointed above) in aqueous solution, the main species formed in the pH-range of interest of natural fluids are the hydrolytic ones. ... 

The amino group now provides the nucleus for purine ring formation, an extended series of reactions we shall not describe. The first-formed purine product is inosine 5 -phosphate (IMP), which leads to either AMP or GMP these require amination at alternative sites, and utilize either GTP- or ATP-dependent reactions for amination. GTP or ATP (as appropriate) will also be required for further phosphorylations to produce the nucleotide triphosphates. 

Subsequent to the preparation of 8, a number of other sapphyrin-nucleobase conjugated carriers were synthesized in the authors laboratories. These include compound 9 that has two cytosine moieties appended to the sapphyrin macrocycle, as well as two guanine-bearing sapphyrins, 10 and 11. Here, as expected, the ditopic sapphyrin-guanine receptor 10 served to enhance the transport of cytosine-5 -monophosphate (but not GMP or AMP). By contrast, the two tritopic receptors, namely 9 and 11, were designed with the intention of allowing for the formation of the complexation-derived triple-helix-like C-G-C and G-C-G motifs in addition to phosphate chelation by sapphyrin. In other words, it was expected that a centrally... 

All biosynthetic pathways are under regulatory control by key allosteric enzymes that are influenced by the end products of the pathways. For example, the first step in the pathway for purine biosynthesis is inhibited in a concerted fashion by nucleotides of either adenine or guanine. In addition, the nucleoside monophosphate of each of these bases inhibits its own formation from inosine monophosphate (IMP). On the other hand, adenine nucleotides stimulate the conversion of IMP into GMP, and GTP is needed for AMP formation. 

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