Inosine monophosphate purine synthesis

Inosine monophosphate dehydrogenase (EVDPDH) is a key enzyme of purine nucleotide biosynthesis. Purine synthesis in lymphocytes exclusively depends on the de novo synthesis, whereas other cells can generate purines via the so-called salvage pathway. Therefore, IMPDH inhibitors preferentially suppress DNA synthesis in activated lymphocytes. 

Inosine monophosphate dehydrogenase (IMPDH) is the key enzyme of purine nucleotide biosynthesis. Proliferation of activated lymphocytes dq ends on rapid de novo production of purine nucleotides for DNA synthesis. 

Mycophenolate mofetil was approved by the FDA in 1995, and enteric-coated mycophenolic acid was approved in 2004. Both agents are considered to be adjunctive immunosuppressants. Mycophenolic acid acts by inhibiting inosine monophosphate deydrogenase, a vital enzyme in the de novo pathway of purine synthesis. Inhibition of this enzyme prevents the proliferation of most cells that are dependent on the de novo pathway for purine synthesis, including T cells.7,11,26-28... 

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.

In the purine nucleotide pathway, the purine nucleotide is synthesised upon the phosphoribose using several small molecules. The first purine nucleotide formed is inosine monophosphate (IMP) it is an intermediate on the pathway for the synthesis of both adenine and guanine nucleotides (Figure 20.8). 

Mycophenolate mofetil has a more specific effect on lymphocytes than on other cells. It inhibits inosine monophosphate dehydrogenase, which catalyzes purine synthesis in lymphocytes. It is used in acute tissue rejection responses. 

The major intermediates in the biosynthesis of nucleic acid components are the mononucleotides uridine monophosphate (UMP) in the pyrimidine series and inosine monophosphate (IMP, base hypoxanthine) in the purines. The synthetic pathways for pyrimidines and purines are fundamentally different. For the pyrimidines, the pyrimidine ring is first constructed and then linked to ribose 5 -phosphate to form a nucleotide. By contrast, synthesis of the purines starts directly from ribose 5 -phosphate. The ring is then built up step by step on this carrier molecule. 

Figure 10-1. Overview of purine synthesis. Details of the first two reactions and sources of the atoms of the purine ring in inosine 5 -monophosphate (IMP) are shown. PRPP, 5 -phosphoribosyl-1-pyrophosphate Gin, glutamine Gly, glycine Asp, aspartate THF, tetrahydrofolate.

Figure 10-2. Regulation of purine synthesis by the nucleotides and the intermediate, 5 -phosphoribosyl-1 -pyrophosphate (PRPP). Both feedback and feed-forward mechanisms are utilized in this intricate scheme. IMP, inosine monophosphate.

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... 

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... 

Unlike these nonspecific agents, mycophenolate mofetil (6.4) tends to be a lymphocyte-specific cytotoxic agent. Mycophenolate mofetil is a semisynthetic derivative of mycophe-nolic acid, isolated from the mold Penicillium glaucum. It inhibits both T and B lymphocyte action. Since it inhibits the enzyme inosine monophosphate dehydrogenase, which catalyses purine synthesis in lymphocytes, this agent has a more specific effect on lymphocytes than on other cell types. Mizoribine (6.5) is a closely related drug which inhibits nucleotide synthesis, preferentially in lymphocytes. 

Mechanism of Action Selectively inhibits inosine monophosphate dehydrogenase in the de novo pathway of purine synthesis, producing potent cytostatic effects on T and B lymphocytes... 

C. Synthesis of inosine monophosphate, the "parent" purine nucleotide... 

Purine synthesis uses a PRPP handle where the ring is assembled to make a 5 NMP, inosine monophosphate (IMP). 

The synthesis of the purine ring is considerably more complex than pyrimidine synthesis. Starting with P-Rib-PP, inosine monophosphate (IMP) is formed in 10 steps (Fig. 15-16). The overall reaction is... 

The pathway to purine synthesis is rather long. Parts of the purine molecule are from glycine, tetrahydrofo-late (a one-carbon donor), glutamine, CO and aspartate (fig. 5.4) (B-3). This pathway enters the purine section of the DNA Funhouse at IMP" (Inosine monophosphate), at the ground level. The body can avoid part of the long... 

Mycophenolate mofetil is the 2-moiphohnoethyl ester of mycophenolic acid (MPA). It is a prodrug that is rapidly hydrolyzed to the active form, mycophenolic acid. Mycophenolic acid is a selective, uncompetitive and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH). IMPDH is an important enzyme in the de novo pathway of purine nucleotide synthesis. This pathway is very important in B and T lymphocytes for proliferation. Other cells can use salvage pathways. Therefore MPA inhibits lymphocyte proliferation and functions. The mofetil ester is first converted to MPA which then is metabolized to an inactive glucuronide (Alhson and Eugui, 2000). MPA has a half-hfe of about 16 hours (Fulton and Markham, 1996). 

FIGURE 9.11 Conversion of inosine monophosphate to adenosine monophosphate and guanos-ine monophosphate. Aspartate donates an amino group in the s)mthesis of AMP. Glutamine donates an amino group in the synthesis of GMP. Folate is not used in these steps of purine metabolism. 

HGPRT Hypoxanthine-guanine phosphoribosyltransferase the enzyme that catalyzes the synthesis of inosine monophosphate (IMP) and guano-sine monophosphate (GMP) from hypoxanthine and guanine, respectively. It makes up part of the purine salvage pathway, a way of recycling purine bases back to the nucleotides. 

The substrate for this reaction, ct-D-ribose-5-phosphate, is a product of the pentose phosphate pathway.) Figure 14.24 illustrates the initial phase in the pathway by which PRPP is converted to inosine monophosphate (inosinate), the first purine nucleotide. The process begins with the displacement of the pyrophosphate group of PRPP by the amide nitrogen of glutamine in a reaction catalyzed by glutamine PRPP amidotransferase. This reaction is the committed step in purine synthesis. The product formed is 5-phospho-/3-D-ribosylamine. 

This produces the first nucleotide in the synthesis of inosine monophosphate (IMP). Note that this is not merely a base, but is also a nucleotide IMP at the crossroads is an important precursor of other purine nucleotides. There are separate pathways to form guanosine monophosphates (GMP) and adenosine monophosphate (AMP). 

Recently, evidence for large-scale associations between enzymes involved in the de novo pathway for purine synthesis has emerged. This pathway involves 10 steps that convert PRPP to inosine monophosphate. In higher eukaryotes, these steps are catalyzed by six enzymes one is trifiinctional and two are bifunctional. Versions of these enzymes tagged with green fluorescent protein (GFP) or orange fluorescent protein (OFP)... 

The de novo pathway of purine synthesis is complex, consisting of 11 steps, and requiring 6 molecules of ATP for every purine synthesized. The precursors that donate components to produce purine nucleotides include glycine, ribose 5-phosphate, glutamine, aspartate, carbon dioxide, and N -formyl FH4 (Fig. 41.1). Purines are synthesized as ribonucleotides, with the initial purine synthesized being inosine monophosphate (IMP). Adenosine monophosphate (AMP) and guano sine monophosphate (GMP) are each derived from IMP in two-step reaction pathways. 

Mechanism of action This drug is rapidly converted into mycophenolic acid, which inhibits inosine monophosphate dehydrogenase, an enzyme in the de novo pathway of purine synthesis. This action suppresses both B and T lymphocyte activation. Lymphocytes are particularly susceptible to inhibitors of the de novo pathway because they lack the enzymes necessary for the alternative salvage pathway for purine synthesis. 

At very low values of EC, when AMP is elevated it is deaminated via AMP deaminase to inosine monophosphate (IMP). This further displaces the adenylate kinase reaction in the direction of ATP synthesis. The IMP is dephosphorylated by nucleotide phosphatase, and the inosine is phosphorylyzed via purine nucleotide phosphorylase, releasing hypoxanthine and ribose 1-phosphate. The latter is metabolized via the pentose phosphate pathway, and most of the carbon atoms enter glycolysis. Because this course of events depletes the overall adenine nucleotide pool, and hence the scope for ATP production in the longer term, it represents a metabolic last ditch stand by the cell to extract energy even from the energy currency itself ... 

Fig. 29.2 Caffeine is produced from xanthosine derived from four routes (1) inosine-5 -monophosphate (IMP) originating from de novo purine synthesis (de novo route), (2) adenosine released from the 5-adenosyhnethionine (SAM) cycle, (3) the cellular adenosine nucleotide pool (AMP route), and (4) the guanine nucleotide pool (GMP route). Enzymes AMPDA AMP deaminase, APRT adenine phosphorihosyltransferase, AK adenosine kinase, ARN adenosine nucleosidase, GRD guanine deaminase, IMPDH IMP dehydrogenase, 5 NT 5 -nucleotidase...

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