Inosine enzymic synthesis

Inosine 5 -(a-D-glucopyranosyl pyrophosphate) (50) was obtained by use of the pyrophosphorylases, effective for the respective guanosine derivative, from animal tissues,50 pea,223 or Arthrobacter vis-cosus.218 The enzymic synthesis of the a-D-glucopyranosyl pyrophosphate esters of 2 -deoxyuridine223 and 2 -deoxyadenosine216 has also been described. 

The reverse phenomenon, decreased enzyme synthesis, can also be the mechanism of drug resistance. The antimetabolite pro-drug 6-mercaptopurine (6MP) is activated to its nucleotide by inosine-5 -phosphate pyrophosphorylase. The enzyme is deleted in resistant neoplastic cells. Resistance to 5-fluorouracil similarly develops by deletion of the enzyme converting this pro-drug to its active nucleotide. A mechanism of resistance by which a drug is excluded from its site of action can also be operative. This has been established for tetracycline antibiotics. Here the permeability of the cellular membrane to the drug is altered so that it cannot penetrate and accumulate within the target cell. Similarly, it has been demonstrated with such a membrane modification in MTX-resistant leukemia cells in mice. 

Howard and Miles (1965) have described the enzymic synthesis of inosine-6- 0, inosine 5 -phosphate-6- 0, and guanosine-6- 0. In the course of this work they used infrared spectroscopy of Dj 0 and Dj 0 solutions to evaluate the conversion of 5 -AMP to 5 -IMP and the conversion of 2,6-diamino-9-/S-D-ribofuranosylpurine to guanosine. Kinetic plots of the data for these reactions were made in the same way as already indicated in Fig. 15.5 of Howard and Miles (1964). Figure 15.6 shows spectra for the enzymic conversion of 2,6-diamino-9-) -D-ribofuranosylpurine to guanosine, and Fig. 15.7 shows a plot of the absorbances of certain specific absorption bands during the course of the reaction. 

Investigations on the reversal of nucleosidase activity established that purified nucleosidases from rat liver could synthesize inosine and guanosine from ribose 1-phosphate and the respective purines 89a), This procedure has been extended to the enzymic synthesis of iV-ribosylnicotinamide 89h) and A-2-deoxyribosylhypoxanthine and -azaguanine 89c) from their respective bases and ribose 1-phosphate. Reactions such as these may play a role in the natural synthesis of nucleotides. 

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... 

The synthesis of inosinic acid (123) from AIR (106) using soluble avian liver enzymes has been shown to proceed in several steps. The first step involves the formation of C-AIR (107) by carboxylation of the aminoimid-azole (106) (Scheme 15) (57JA1511). 

Mycophenolate sodium (62 Myfortic Norvatis, 2003) is an immunosuppressant drug used to prevent rejection in organ transplantation. It is a selective, noncompetitive, reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme in the de novo pathway of guanosine nucleotide synthesis. Thus, mycophenolic acid (61), originally... 

The de novo synthesis of inosinic acid The salvage pathways Purine nucleotide interconversions Other enzymes... 

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... 

With over-expressed fluorinase and under optimised reaction conditions, a synthesis of [ F]-FDA 5d from [ F]-fluoride was achieved in a radiochemical yield (RCY) of 95%. Also in coupled enzyme systems, where the fluorinase is coincubated with other enzymes, the syntheses of [ F]-5 -fluoro-5 -deoxy-inosine... 

Mechanism of Action An immunologic agent that suppresses the immunologically mediated inflammatory response by inhibiting inosine monophosphate dehydrogenase, an enzyme that deprives lymphocytes of nucleotides necessary for DNA and RNA synthesis, thus inhibiting the proliferation of T and B lymphocytes. Therapeutic Effect Prevents transplant rejection. 

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. Mycophenolate mofetil inhibits a specific enzyme (inosine monophosphate dehydrogenase) that is responsible for the synthesis of DNA precursors in T and B lymphocytes.39 50 Because these lymphocytes cannot synthesize adequate amounts of DNA, their ability to replicate and proliferate is impaired, thus blunting the immune response. This drug may also inhibit lymphocyte attraction and adhesion to the vascular endothelium, thereby impairing the lymphocytes ability to migrate to the site of the foreign (transplanted) tissues and to infiltrate from the bloodstream into these tissues.50... 

A few steps convert inosinate into either AMP or GMP (Figure 25,9). Adenylate is synthesized from inosinate by the substitution of an amino group for the carbonyl oxygen atom at C-6. Again, the addition of aspartate followed by the elimination of fumarate contributes the amino group. GXP, rather than ATP, is the phosphoryl-group donor in the synthesis of the adenylosuccinate intermediate from inosinate and aspartate. In accord with the use of GTP, the enzyme that promotes this conversion, adenylsuccinate synthase, is structurally related to the G-protein family and does not contain an ATP-grasp domain. The same enzyme catalyzes the removal of fumarate from adenylosuccinate in the synthesis of adenylate and from 5-aminoimidazole-4-jV-succinocarboxamide ribonucleotide in the synthesis of inosinate. 

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). 

Mercaptopurine is not active until it is anabolized to the phosphorylated nucleotide. In this form, it comgietes with endogenous ribonucleotides for enzymes that convert ino-sinic acid into adenine- and xanthinc-ba.sed ribonucleotides. Furthermore, it is incoiporated into RNA. where it inhibits further RNA synthesis. One of its main metabolites is 6-mcthylmercaptopurine ribonucleotide, which also is a potent inhibitor of the conversion of inosinic acid into purines. - "... 

The answer is c, (Murray, pp 375-801. Scriver, pp 2513-2570. Sack, pp 121-138. Wilson, pp 287-320.) 5 -phosphoribosyl-l-pyrophosphate (PRPP) donates the ribose phosphate unit of nucleotides and is absolutely required for the beginning of the synthesis of purines. In fact, the enzymes regulating the synthesis of PRPP and the subsequent synthesis of phospho-ribosylamine from PRPP are all end product-inhibited by inosine... 

---