Inosines. additions

On silylation-amination of the disodium salts of inosine-5 -phosphate 238a or of guanosine-5 -phosphate 238 b with benzylamine, the phosphate moieties are also transiently protected during amination by silylation (cf also the silylation of uridine-5 -phosphate 224) to give, after transsilylation with methanol and addition of NaOH, the desired sodium salt of N -benzyladenosine-5 -phosphate 239a in 80% yield and the sodium salt of the 2-amino derivative 239 b in 78% yield [64] (Scheme 4.23). 

HPLC coupled to MS was used for the determination of dimethyl xanthine metabolites in plasma.82 There have also been a number of methods published on the use of HPLC with a PDA detector. In 1996, Mei published a method for the determination of adenosine, inosine, hypoxanthine, xanthine, and uric acid in microdialysis samples using microbore column HPLC with a PDA detector.63 In this method, samples were directly injected onto the HPLC without the need for any additional sample treatment. 

AZT works by specifically blocking DNA synthesis carried out by HIV reverse transcriptase. Other related compounds are also being tested to see if they specifically affect HIV reverse transcriptase. Such compounds might have equivalent antiviral effects. If they have fewer side effects than AZT, they may be even more effective in treating HIV-infected individuals. Two additional antivirals related to AZT have recently been approved for anti-HIV therapy, dideoxy-inosine (DDI) and dideoxycytosine (DDC). These drugs are predominantly recommended for individuals who cannot tolerate AZT, or for whom AZT has ceased to be effective although they are effective against HIV, they do have side effects. Nevertheless, they may be important because AZT does not indefinitely reduce the amount of virus in HIV-infected individuals. 

With [ N2]hydrazinium hydrogen sulfate and potassium hydroxide, the 2, 3, 5 -tri-0-acetyl-l-( N-amino) (3- N) inosine 54 is obtained (Scheme III.29). The reaction follows the same reaction pathway as described in Scheme III.28 addition of the nucleophile at C-6, ring opening between C(6) and N(l), and ring closure with elimination of nitrous oxide and water. This Sn(ANRORC) reaction provides us with an good entry to N-ring-labeled purines. 

The first surprise was that these molecules are much longer than seems necessary for the formation of adapters. In addition, 10-20% of their bases are modified greatly from their original form.171 Another surprise was that the anticodons are not all made up of "standard" bases. Thus, hypoxanthine (whose nucleoside is inosine) occurs in some anticodons. Conventional "cloverleaf" representations of tRNA, which display their secondary structures, are shown in Figs. 5-30 and 29-7. However, the molecules usually have an L shape rather than a cloverleaf form (Figs. 5-31 and 29-6),172 and the L form is essential for functioning in protein synthesis as indicated by X-ray and other data.173 Three-dimensional structures, now determined for several different tRNAs,174 175 are all very similar. Structures in solution are also thought to be... 

The existence of two separate enzymes in animal tissues responsible for the liberation of ammonia from each of the two aminopurines, adenine and guanine, the latter specific for the free purine and the former for the nucleosides, was initially presented by Jones and his colleagues 11, 12). In 1928, Schmidt 13-15) demonstrated that AMP aminohy-drolase was responsible for the appearance of inosinic acid in muscle and for at least a portion of ammonia liberated during contraction. He showed not only a marked specificity for deamination of 5 -AMP but also provided the first clue that muscle adenylic acid (5 -AMP) and yeast adenylic acid (3 -AMP) were different compounds. Initial evidence for guanine and adenosine aminohydrolase including aspects of the specificity were also described by Schmidt 16). Additional details regarding development of interest in purine aminohydrolases are available in several excellent reviews 17-20). 

The pathway from PRPP to the first complete purine nucleotide, inosine monophosphate (IMP), involves 10 steps and is shown in figure 23.10. It would seem logical that the purine should be built up first, followed by addition of ri-bose-5-phosphate, but this is not the case. The starting point is PRPP, to which the imidazole ring is added the six-member ring is built up afterward. 

Inosine formed by either route is then phosphorolyzed to yield hypoxanthine. Although, as we have previously seen, much of the hypoxanthine and guanine produced in the mammalian body is converted to IMP and GMP by a phosphoribosyltransferase, about 10% is catabolized. Xanthine oxidase, an enzyme present in large amounts in liver and intestinal mucosa and in traces in other tissues, oxidizes hypoxanthine to xanthine, and xanthine to uric acid (see fig. 23.20). Xanthine oxidase contains FAD, molybdenum, iron, and acid-labile sulfur in the ratio 1 1 4 4, and in addition to forming hydrogen peroxide, it is also a strong producer of the superoxide anion 02, a very reactive species. The enzyme oxidizes a wide variety of purines, aldehydes, and pteridines. 

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. 

Systemically, in addition to a direct A2AR-mediated vasodilatation, adenosine causes vasoconstriction of isolated perfused arterioles from the hamster cheek pouch, which is often masked by vasodilatation (Doyle et al. 1994 Shepherd et al. 1996). This response to adenosine and its metabolite, inosine, was found to be due to stimulation of perivascular mast cells and the subsequent release of vasoconstrictors. 

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. 

Position of Bond Cleavage PNP (EC 2.U.2.1) from human erythrocytes (homogeneous, purified by formycin B affinity chromatography) as well as from E, coli were allowed to equilibrate a mixture of R-l-[1 0lj]-P, pl Oij, hypoxanthine and inosine at pH 7-00 in 10 mm NMR tubes. The chemical shift differences of the 31p nuclei of the two R-l-P s (13 9 Hz for the human erythrocytic and 13.1 Hz for the E. coli enzyme) as well as of the two P3 resonances (13 9 Hz for erythrocytic and 13-7 Hz for E. coli source) clearly indicated C-0 bond cleavage by these enzymes as well. In addition, no evidence was found over the time course of the NMR measurements (l hr) for purine nucleoside phosphorylase catalyzed exchange of pl o + H2O (solvent) J randomized P. Therefore,... 

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