A -Methyladenine

Conversion of 4-aminopyrazolo [3,4-d] pyrimidine (VIII) to its ribonucleotide by mouse tumours and host tissues has been observed [118,119]. Although no evidence of the anabolism of A -methyladenine (111) [120] to the ribonucleotide was obtained in mice with Ehrlich ascites carcinoma [121, 122], it is anabolized by bacteria [123. 124] and the enzyme responsible was partially purified from Salmonella typhimurium [125]. Human epidermoid carcinoma No. 2 cells resistant to 2-fluoroadenine (H.Ep.-2/FA) have lost adenine phosphoribosyl-... 

The literature on xanthine oxidase [84] and its companion catabolic enzyme uricase [87] has been extensively reviewed. Many purine analogues, with the exception of most 9-substituted purines [262], serve as substrates for xanthine oxidase both in vitro and in vivo, and if the product is a substrate for uricase, in species that possess this enzyme, the ultimate product is allantoin (LVIII). Thus 2-aminoadenine [5], A -methyladenine [122], and purine [129] are all catabolized... 

Fluoroadenine, 2-chloroadenine, 2-aminoadenine, 2- and 8-aza-adenines, 4-aminopyra7.olo[3, 4-d] pyrimidine and 6-methylpurine are converted to their ribonucleosidesby adenosine phosphoribosyltransferase,and their ribonucleosides are converted to the ribonucleotides by adenosine kinase most of the ribonucleotides are then converted to the di- and triphosphates. A -Aminoadenine, A -hydroxyadenine, A -methyladenine, purine, 7-deaza-adenine, and 7-amino-pyrazolo[4, 3-d] pyrimidine are either not substrates or are very poor substrates for the phosphoribosyltransferase, but their ribonucleosides are excellent substrates for the kinase. The ribonucleotides of purine, 7-deaza-adenine and... 

They then prepared the completely methylated derivatives of adeno-sine<8. and guanosine by simultaneous deacetylation and methylation of the acetylated nucleosides. In this way, trimethyl-JV-methyl adenosine and trimethyl-iV-methyl guanosine were formed, and isolated as the hydrochlorides. On hydrolysis of the adenosine derivative by means of dilute hydrochloric acid, 6-A -methyladenine and trimethyl-D-ribo-furanose were isolated. The same trimethyl sugar was isolated from the methylated guanosine and was identified in each case by oxidation, first to trimethyl-7-D-ribonolactone and then to meso-dimethoxy succinic acid. It follows that the sugar component has the furanose ring-structure. 

Polarized IR spectra in PVA film, and the calculated (SCRF) spectra of 7-methyl- and 9-methyladenine, 17 and 18 (R = Me) respectively, have demonstrated that they exist as the amino forms shown [97JPC(A)4361],... 

The bulk effect of water as a solvent is rather dramatic since it causes a drastic reduction of the nucleophilicity of 9-methyladenine N1 and even more of 9-methylguanine 06. As a result, there is a reversal of the nucleophilicity order of the purine bases passing from gas phase to aqueous solution. In fact, in solution, methyladenine is more nucleophilic than methylguanine. Moreover, oxygen and N7 nucleophilic centers of 9-methylguanine compete almost on the same footing in solution (Table 2.2) and also the reactivity gap between N1 and N7 of 9-methyladenine is highly reduced in comparison to the gas phase. 

TABLE 2.3 Stability of the QM-9-Methyladenine and QM-9-Methylguanine Adducts (QM-A and QM-G, Respectively) Relatively to Reactants in the Gas Phase (AGgas) and in Aqueous Solution (AGaq). Activation Energies for Their Decomposition into Free QM and Cytosine (AG e )... 

Although demethylation, which occurs in the liver, is normally considered to be a catabolic process, it may result in conversion of an inactive form of a drug to the active form. Thus 6-(methylthio)purine (XXXIX) is demethylated by the rat to 6-mercaptopurine [205]. This demethylation occurs in the liver micro-somes and is an oxidative process which converts the methyl group to formaldehyde [204, 207]. The 1-methyl derivative of 4-aminopyrazolo[3,4-d] pyrimidine (XLI) is demethylated slowly, but 6-mercapto-9-methylpurine (XLII) not at all [208]. The A -demethylation of puromycin (XLlIl) [209, 210], its aminonucleoside (XLIV) [211], and a number of related compounds, including V-methyladenine and V,V-dimethyladenine, occurs in the liver microsomes of rodents [212]. In the guinea-pig the rate-limiting step in the metabolism of the aminonucleoside appears to be the demethylation of the monomethyl compound, which is the major urinary metabolite [213]. The relationship of lipid solubility to microsomal metabolism [214], and the induction of these demethylases in rats by pre-treatment with various drugs have been studied [215]. 

A further example of interest reported by Etter and co-workers involved the base pair consisting of the pyrimidine 1-methylthymine T and the purine 9-methyladenine A [281. These are derivatives of the base pairs found in the... 

Dimethyl sulfate induced mutation in bacteria and DNA damage in prophage. It forms a variety of alkylated bases, including 7/7-mcthylguanine, 7/3-methyladenine and 7/7-methyladcninc with DNA in vitro. 

A very different but equally direct mechanism is used to repair 1-methyladenine and 3-methylcytosine. The amino groups of A and C residues are sometimes methylated when the DNA is single-stranded, and the methylation directly affects proper base pairing. In E. coli, oxidative demethylation of these alkylated nucleotides is mediated by the AllcB protein, a member of the a-ketoglutarate-Fe2+-dependent dioxygenase superfamily (Fig. 25-27). (See Box 4-3 for a description of another member of this enzyme family.)... 

FIGURE 25-27 Direct repair of alkylated bases by AlkB. The AlkB protein is an a-ketoglutarate-Fe2+-dependent dioxygenase. It catalyzes the oxidative demethylation of 1-methyladenine and 3-methyl-cytosine residues. 

Begley, T.J. Samson, L.D. (2003) AlkB mystery solved oxidative demethylation of Nl-methyladenine and N3-methylcytosine adducts by a direct reversal mechanism. Trends Biochem. Sci. 28, 2-5. 

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