Xanthosine

Tire tautomerism and ionization of xanthosine (21), a 9-substituted xanthine, have been studied by IR spectroscopy in aqueous solution [83MI(2)231].Tlie diketo structure 21 was shown to exist below pH 5, and the 2-enolate anion 22 at neutral and slightly basic pH. 

By analogy with the conversion of uridine 1 into cytidines 6, the conventional amination of inosine 235a, guanosine 235b, or xanthosine 235c and their 2 -deoxy analogues to the adenosines 237 requires ... 

Seasonal variations in the metabolic fate of adenine nucleotides prelabelled with [8—1-4C] adenine were examined in leaf disks prepared at 1-month intervals, over the course of 1 year, from the shoots of tea plants (Camellia sinensis L. cv. Yabukita) which were growing under natural field conditions by Fujimori et al.33 Incorporation of radioactivity into nucleic acids and catabolites of purine nucleotides was found throughout the experimental period, but incorporation into theobromine and caffeine was found only in the young leaves harvested from April to June. Methy-lation of xanthosine, 7-methylxanthine, and theobromine was catalyzed by gel-filtered leaf extracts from young shoots (April to June), but the reactions could not be detected in extracts from leaves in which no synthesis of caffeine was observed in vivo. By contrast, the activity of 5-phosphoribosyl-1-pyrophosphate synthetase was still found in leaves harvested in July and August. 

SWV has been applied to study electrode reactions of miscellaneous species capable to form insoluble salts with the mercury electrode such as iodide [141,142], dimethoate pesticide [143], sulphide [133,144], arsenic [145,146], cysteine [134, 147,148], glutathione [149], ferron (7-iodo-8-hydroxyquinolin-5-sulphonic acid) [150], 6-propyl-2-thiouracil (PTU) [136], 5-fluorouracil (FU) [151], 5-azauracil (AU) [138], 2-thiouracil (TU) [138], xanthine and xanthosine [152], and seleninm (IV) [153]. Verification of the theory has been performed by experiments at a mercury electrode with sulphide ions [133] and TU [138] for the simple first-order reaction, cystine [134] and AU [138] for the second-order reaction, FU for the first-order reaction with adsorption of the ligand [151], and PTU for the second-order reaction with adsorption of the ligand [137]. Figure 2.90 shows typical cathodic stripping voltammograms of TU and PTU on a mercuiy electrode. The order of the... 

Human type II inosine monophosphate dehydrogenase catalyses NAD-dependent conversion of inosine monophosphate (IMP) into xanthosine monophosphate (XMP) measurements of the primary kinetic isotope effect using [ H]IMP suggest that both substrates (IMP and NAD) can dissociate from the enzyme-substrate complex therefore, the kinetic mechanism is not ordered. NMR studies indicate hydride transfer to the B or pro-S face of the nicotinamide ring of NAD, while kinetic studies suggest... 

The second step in the conversion of inosinic acid to guanylic acid is the aminolysis of xanthylic acid with glutamine by xanthosine-5 -phosphate aminase [65]. This aminase, isolated fromf. coli B, is inhibited allosterically by adenosine and adenylic acid [320], and it is also inhibited by psicofuranine (9-0-D-psicofuranosyladenine) (LXXIV) [284, 321-326], which apparently is not... 

The related antibiotic decoyinine (angustmycin A, LXXIII) [285] also inhibits xanthosine-5 -phosphate aminase [329], presumably in the same manner as psicofuranine (LXXIV), but less effectively. There is evidence that psicofuranine (LXXIV) and decoyinine (LXXIII) may be interconvertible in cells [329]. 

GMP synthetase [EC 6.3.4.1], also known as xanthosine-5 -phosphate ammonia ligase, catalyzes the reaction of ATP with xanthosine 5 -phosphate and ammonia to produce GMP, AMP, and pyrophosphate (or, diphosphate). GMP synthetase (glutamine-utilizing) [EC 6.3.5.2] catalyzes the reaction of ATP with xanthosine 5 -phosphate, L-glutamine, and water to produce GMP, AMP, L-glutamate, and pyrophosphate. 

This enzyme [EC 3.5.4.15], also known as guanosine aminohydrolase, catalyzes the hydrolysis of guanosine to yield xanthosine and ammonia. 

Purines, pyrimidines, and the corresponding nucleosides have been separated in reversed phase systems. The five mqior nucleosides as well as inosine and xanthosine and the corresponding seven bases were separated by Hartwick and Brown (405). The selective determination of adenosine in the presence of other serum components has been reported (406). 

Accumulation of Cu(II) complexes with xanthine and xanthosine has also been utilized in stripping analysis [74]. Copper(II) indapamide complex was adsorptively accumulated at a HMDE and used for the determination of the ligand in the cathodic stripping step [75]. 

Xanthine and xanthosine were investigated on HMDE, applying out-of-phase ac and dc voltammetries [74]. It has been shown that both compounds are strongly adsorbed and interact chemically. In the cathodic stripping process, one could determine both compounds at trace level. Naidu et al. [146] have performed polaro-graphic studies to show that the product of anodic reaction (prewave) of potassium isobutyl xanthate is strongly adsorbed at the mercury electrode. 

There are many examples of each of these possibilities (see figure 8.8). In the case of the 2 sugar position, most of the 2 -substititutions have been achieved with 2 -0-alkyl, 2 -amino, and 2 -fluoro replacements. Purine base substitutions have been performed with 2-aminopurine, xanthosine, and isoguanosine. Phosphodiester replacements have been effected using phosphorothioate substitutions. As for the non-nucleotide linkers, propanediol linkers have been employed. Using these various bioisosteric substitutions,... 

IMP, is synthesised with mutants of Bacillus subtilis or Corynehacterium am-moniagenes. Xanthosine 5 -monophosphate is produced with Corynebacte-rium or Bacillus and subsequently converted into GMF by Bacillus and other strains [6]. Alternatively, another related compound, 5 -amino-4-imidazole carboxamide-l-riboside-5 -phosphate, is produced by Bacillus megaterium and chemically converted into GMP [22, 36],... 

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