Nucleoside monophosphate kinase phosphorylase

Pyrimidine bases are normally salvaged by a two-step route. First, a relatively nonspecific pyrimidine nucleoside phosphorylase converts the pyrimidine bases to their respective nucleosides (Fig. 41.17). Notice that the preferred direction for this reaction is the reverse phosphorylase reaction, in which phosphate is being released and is not being used as a nucleophile to release the pyrimidine base from the nucleoside. The more specific nucleoside kinases then react with the nucleosides, forming nucleotides (Table 41.2). As with purines, further phosphorylation is carried out by increasingly more specific kinases. The nucleoside phosphorylase-nucleoside kinase route for synthesis of pyrimidine nucleoside monophosphates is relatively inefficient for salvage of pyrimidine bases because of the very low concentration of the bases in plasma and tissues. 

The salvage activities of T. foetus and T. vaginalis also differ (22,77). Unlike T. foetus, the level of uracil PRTase activity is very low. Uracil is converted into uridine by a uridine phosphorylase uridine is then phosphorylated by a uridine phosphotransferase to UMP (Fig. 6.15). Cytidine and thymidine also are converted into their nucleotide monophosphates by phosphotransferase activities. There is no detectable pyrimidine nucleoside kinase activity and the only significant interconversion among salvaged pyrimidines is catalyzed by cytidine deaminase to form uridine. 

Fig. 41.10. Salvage of bases. The purine bases hypoxanthine and gnanine react with PRPP to form the nucleotides inosine and gnanosine monophosphate, respectively. The enzyme that catalyzes the reaction is hypoxanthine-gnanine phosphoribosyltransferase (HGPRT). Adenine forms AMP in a reaction catalyzed by adenine phosphoribosyltransferase (APRT). Nucleotides are converted to nucleosides by 5 -nucleotidase. Free bases are generated from nncleosides by purine nucleoside phosphorylase. Deamination of the base adenine occurs with AMP and adenosine deaminase. Of the purines, only adenosine can be directly phosphorylated back to a nucleotide, by adenosine kinase.

As shown in Table 4, among the series of 2 -deoxy-2 -methylideneuridines 23, only the 5-methyl derivative 23f showed marginal activity against L1210 cells. In contrast with 2 -deoxy-5-fluorouridine, 5-fluoro derivative 23b was completely inactive at up to 100 Xg/mL, probably due to inability of the latter to act as a substrate of nucleoside kinases. As 5-FU is an effective inhibitor of the growth of various tumor cells, this result may imply that 23b could not be a substrate for nucleoside phosphorylase. The ineffectiveness of the 5-ethynyl derivative 23i would also be related to insusceptibility to nucleoside kinase, because 2 -deoxy-5-ethynyluridine 5 -monophosphate is known to be a potent inhibitor of thymidylate synthetase. ... 

Thus the utilization of exogenous purine nucleosides involves three phosphoribosyltransferases and one nucleoside phosphorylase, if a phosphorolytic cleavage of the nucleosides is the first reaction to take place. An alternative pathway for the utilization of nucleosides would be a direct phosphorylation to monophosphates. The question is then Do such nucleoside kinases exist in E.coli,and if this is the case, how many enzymes are involved ... 

---