Inosinic acid synthesis

Azathioprine acts through its major metabolite, 6-thioguanine. 6-Thioguanine suppresses inosinic acid synthesis, -cell and T-cell function, immunoglobulin production, and interleukin-2 secretion (see Chapter 55). 

When a comparison was made of inosinic acid synthesis from glycine-1-C and HC OOH in the presence of added nonlabeled inosinic acid, it was observed that radioactive formate was incorporated into the acid far in excess of that anticipated from the amount of glycine utilized.i " For example, instead of a 2 1 molecular utilization of the formate glycine... 

B. Inosinic Acid Synthesis in Pigeon Liver—Detailed Reaction... 

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). 

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

Azathioprine is a cytotoxic inhibitor of purine synthesis effective for the control of tissue rejection in organ transplantation. It is also used in the treatment of autoimmune diseases. Its biologically active metabolite, mercaptopurine, is an inhibitor of DNA synthesis. Mercaptopurine undergoes further metabolism to the active antitumour and immunosuppressive thioinosinic acid. This inhibits the conversion of purines to the corresponding phosphoribosyl-5 phosphates and hypoxanthine to inosinic acid, leading to inhibition of cell division and this is the mechanism of the immunosuppression by azathioprine and mercaptopurine. Humans are more sensitive than other species to the toxic effects of the thiopurines, in particular those involving the haematopoietic system. The major limiting toxicity of the thiopurines is bone marrow suppression, with leucopenia and thrombocytopenia. Liver toxicity is another common toxic effect. 

ThiolMP and ThioGMP are feedback inhibitors of phosphoribosylpyrophosphate amido-transferase, which is the first, and rate-limiting step in the synthesis of purine. In addition, these analogs inhibit the de novo biosynthesis of purine and block the conversion of inosinic acid to adenylic acid or guanylic acid. The triphosphate nucleotides are incorporated into DNA, and this results in delayed toxicity after several cell divisions. 

When injected, azathioprine (Imuran) is rapidly converted to 6-mercaptopurine. The half-life of azathioprine after intravenous injection is 10 to 20 min, and that of 6-mercaptopurine is somewhat longer. The cytotoxic activity of these thiopurines is due to the conversion of mercaptopurine to 6-thiouric acid, a noncarcinostatic metabolite. This action is thought to block the excess synthesis of inosinic acid from its precursors, glutamine and phosphoribosylpyrophosphate. In addition, unlike cyclophosphamide, azathioprine is a potent anti-inflammatory substance that can cause a reduction in the number of monocytes and neutrophils at inflammatory sites. Antibody responses are also inhibited by azathioprine. Studies in humans have shown that azathioprine decreases the y-globulin and antibody levels, thus influencing IgG rather than IgM production. This makes azathioprine an effective immunosuppressant in the early phases of immune responses. It is less effective or completely ineffective in altering either the effector phase or already established reactivities. 

Finally, muscle inosinic acid itself was synthesized by Levene and Tipson. This was the first (partial) synthesis of a naturally occurring nucleotide. Phosphorylation of 2,3-isopropylidene-inosine, the structure of which has already been discussed, gave the corresponding 5-phospho derivative, from which the isopropylidene group was cautiously hydrolyzed, yielding 5-phosphoinosine which proved to be identical with muscle inosinic acid. 

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. - "... 

Purine nucleotides can be produced by two different pathways. The salvage pathway utilizes free purine bases and converts them to their respective ribonucleotides by appropriate phosphoribosyltransferases. The de novo pathway utilizes glutamine, glycine, aspartate, N -formyl FH4, bicarbonate, and PRPP in the synthesis of inosinic acid (IMP), which is then converted to AMP and GMP. 

The first purine nucleotide synthesized is inosinic acid (IMP), which is the precursor for the synthesis of... 

HGPRT is responsible for the conversion of guanine to guanylic acid and hypoxanthine to inosinic acid. These two conversions require PRPP as the cosubstrate and are important reutilization reactions involved in the synthesis of nucleic acids. A deficiency in the HGPRT enzyme leads to increased metabolism of guanine and hypoxanthine to uric acid, and more PRPP to interact with glutamine in the first step of the purine pathway." Complete absence of HGPRT results in the childhood Lesch-Nyhan syndrome, characterized by choreoathetosis, spasticity, mental retardation, and markedly excessive production of uric acid. A partial deficiency of the enzyme may be responsible for marked hyperuricemia in otherwise normal, healthy individuals. 

Ribavirin monophosphate competitively inhibits cellular inosine-5 -phosphate dehydrogenase and interferes with the synthesis of guanosine triphosphate (GTP) and, thus, nucleic acid synthesis in general. Ribavirin triphosphate also competitively inhibits the GTP-dependent 5 -capping of viral messenger RNA and, specifically, influenza virus transcriptase activity. Ribavirin appears to have multiple sites of action, and some of these (e.g., inhibition of GTP synthesis) may potentiate others (e.g., inhibition of GTP-dependent enzymes). 

Purine Synthesis from PRPP to Inosinic Acid (Figure 22.4, Figure 22.5)... 

Synthesis of ATP and GTP from Inosinic Acid (Figure 22.6, Diagram 1, 2)... 

Ribarvirin is a broad-spectmm antiviral agent that inhibits viral mRNA synthesis (Section 27.10). A step in the metabolic pathway responsible for the synthesis of guanosine triphosphate (GTP) converts inosine monophosphate (IMP) into xanthosine monophosphate (XMP). Ribarvirin is a competitive inhibitor of the enzyme that catalyzes that step. Thus, ribarvirin interferes with the synthesis of GTP and, therefore, with all nucleic acid synthesis. 

It also mimics inosinic acid thereby causing a negative feedback suppression of the synthesis of inosinic acid. It has been observed that a portion of the drug gets converted to thioguanine, which is ultimately incorporated into both DNA and RNA to give rise to the formation of defective nucleic acids. In this manner the synthesis and functionalities of the resulting nucleic acid are impaired in various ways. It finally helps in the inhibition of cell mitosis. 

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