Guanine utilization

Schwesingefs phosphazene base 2-tert-butylamino-2-diethylamino-l,3-dimethyl-perhydro-l,3,2-diazaphosphorine (PS-BEMP has a pKb = 27.5 in MeCN) has been immobilized and shown to have immense utility in the N- and O-alkylation of many weakly acidic heterocycles. Kim et al. has made extensive use of this reagent in the multi-step synthesis of a small collection of guanines possessing potential antiviral activity [90]. The generic procedure involved the direct alkylation of the purine moiety (20) (Scheme 2.64), promoted by PS-BEMP, resulting in a mixture... 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

The reported strategies utilized in DNA sensing include (1) sequence-specific hybridization processes based on the oxidation signal of most electroactive DNA bases, guanine and adenine [13,24] or (2) quasi-specific detection of small molecules capable of binding by intercalation or complexation with DNA, such as metal coordination complexes, antibiotics, pesticides, pollutants, etc. [17,18] or in the presence of some metal tags such as gold, silver nanoparticles, etc. [23,50,51]. 

When ATP releases inorganic phosphate and reverts to ADP, a quantity of energy equivalent to 31 kJ of energy per mole of ATP is released that can be utilized metabolically. A pair of species that are similar in function to ATP and ADP are guanine triphosphate (GTP) and guanine diphosphate (GDP). 

The enzyme hypoxanthine/guanine phosphoribosyltransferase (HG-PRTase) is an example of an enzyme activity that can utilize either of two substrates. In this case, both hypoxanthine and guanine can be acceptors of the phosphate donated by phosphoribosyl pyrophosphate (PRibPP). Since it is possible to separate hypoxanthine from guanine and the IMP from GMP, it is possible to study the utilization of one substrate in the presence of the other, a condition that parallels that expected in a cell. 

Figure 10.6 HPLC elution profiles of an incubation mixture made up of. 2 nmol of hypoxanthine/guanine phosphoribosyltransferase, SO fiM guanine (G), SO /xM hypo-xanthine (H), 100 fiM PRibPP, and 1 mM MgCl2 in potassium phosphate (pH 7.4). At time intervals of 0 to 5 minutes, aliquots of the mixture were injected onto the HPLC ion-exchange column and eluted. Inset Time-dependent utilization of H and G and formation of GMP and IMP as determined by the absorbance of each peak at 254 nm. (From Ali and Sloan, 1982.)...

The results of an experiment are shown in Figure 10.6. The formation of GMP and IMP from guanine and hypoxanthine, respectively, can be followed. With this method it was possible to track the initial rates of formation of IMP and GMP separately, the initial rates of both determined simultaneously, and the rate of PRibPP utilization with a fixed ratio of hypoxanthine and guanine. 

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