Guanosine phosphorylase

Guanosine is degraded in a two-step reaction sequence. First, guanosine phosphorylase phosphorolyses the nucleoside to free guanine and ribose-1-phosphate. 

They did find that these compounds behaved kinetically as competitive inhibitors of polymerization of the normal substrates e.g., guanosine 5 -diphosphate. These authors suggested that the successful completion of the polynucleotide phosphorylase reaction requires that the nucleotide be capable of assuming the anti conformation. Also, Kapuler and Reich (53) have found that both 8-bromo- and 8-oxoguanosine 5 -triphosphates are very poor substrates in the E. coli RNA polymerase reaction and are competitive inhibitors with respect to guanosine 5 -triphosphate as a substrate. 

Purine nucleoside phosphorylase converts inosine and guanosine into their respective purine bases, hypoxanthine and guanine. 

The conversions of inosine to hypoxanthine (Fig. 25-17, step e), of guanosine to guanine (step g), and of other purine ribonucleosides and deoxyribonucleo-sides to free purine bases are catalyzed by purine nucleoside phosphorylase.318 321b Absence of this enzyme also causes a severe immune deficiency which involves the T cells. However, B cell function is not impaired.312 315 322... 

A guanosine aminohydrolase preparation from Pseudomonas convexa No. 149 free from guanine, adenine and adenosine aminohydrolase and nucleoside phosphorylase activities has been described (63). Of the 22... 

After purine nucleotides have been converted to the corresponding nucleosides by 5 -nucleotidases and by phosphatases, inosine and guanosine are readily cleaved to the nucleobase and ribose-1-phosphate by the widely distributed purine nucleoside phosphorylase. The corresponding deoxynucleosides yield deoxyribose- 1-phosphate and base with the phosphorylase from most sources. Adenosine and deoxyadenosine are not attacked by the phosphorylase of mammalian tissue, but much AMP is converted to IMP by an aminohydrolase (deaminase), which is very active in muscle and other tissues (fig. 23.20). An inherited deficiency of purine nucleoside phosphorylase is associated with a deficiency in the cellular type of immunity. 

A close look at this reaction reveals that in the opposite direction, the reaction is of the phosphorolysis type. For this reason, the enzymes catalyzing the reaction with ribose-l-phosphate are called phosphorylases, and they also participate in nucleic acid degradation pathways. Purine nucleoside phosphorylases thus convert hypoxanthine and guanine to either inosine and guanosine if ribose-l-phosphate is the substrate or to deoxyinosine and deoxyguanosine if deoxyribose-1-phosphate is the substrate. Uridine phosphorylase converts uracil to uridine in the presence of ribose-l-phosphate, and thymidine is formed from thymine and deoxyribose-l-phosphate through the action of thymidine phosphorylase. 

Catabolism of the nucleotides (Figure 24-3, B) begins with removal of their ribose-linked phosphate, a process catalyzed by purine 5 -nucleotidase. Removal of the ribose moiety of inosine and guanosine by the action of purine-nucleoside phosphorylase forms hypoxanthine and guanine, both of which are converted to xanthme. Xanthine is converted to uric acid through the action of xanthine oxidase. 

The phosphorylase can catalyze the formation of inosine or deoxyinosine, and of guanosine or deoxyguano-sine, but not adenosine or deoxyadenosine. However, the last two nucleosides can be converted to inosine and deoxyinosine by adenosine deaminase. The normal function of the phosphorylase appears to be the formation... 

Purine nucleosides are cleaved by the action of purine nucleoside phosphorylase with the liberation of ribose 1-phosphate (Kl, PI). The enzyme is apparently specific for purines. The material from erythrocytes catalyzes the phosphorolysis of purine but not pyrimidine nucleosides (T6.) Purine phosphorylase activity is found widespread in nature and in many animal tissues (FIO). Friedkin and Kalckar investigated an enzyme capable of cleaving purine deoxynucleosides to the aglycone and deoxy-ribose 1-phosphate. They concluded that the enzyme was identical to that which splits purine ribonucleosides (F8, F9). This enzyme is capable of degrading inosine, xanthosine, and guanosine to forms readily attacked by other enzymes. In so doing, it permits living cells to retain the ribose and deoxyribose moieties. 

This concept has been extended. Thus the trione (696) rapidly and irreversibly inactivates human erythrocyte nucleoside phosphorylase (PNPase), which catalyzes the reversible phosphorylation of inosine and guanosine to the respective bases and ribose 1-phosphate. Inhibitors of this enzyme have several potential medical applications, for example, in the prevention of foreign tissue rejection, in the treatment of gout and malaria, and for the potentiation of antineoplastic nucleosides. Mechanistically the 5,8-dione (quinone) (696) enters the enzyme active site. An active-site nucleophilic residue subsequently converts the quinone moiety to a hydroquinone by reductive addition (701). The resulting hydroquinone affords an alkylating quinone methide species by elimination of HCl (702) and then traps a second nucleophilic enzyme residue by a Michael type reaction (703). Cross-linking of the active site rationalizes the observed potency <91B8480>. 

Phosphoribosyltransferase activity was found for hypoxanthine, guanine and xanthine but not for adenine (2). Adenine and guanine deaminase activities are present. Phosphorylase activities were found for adenosine, guanosine and inosine. Also present were adenosine kinase and a guanosine phosphotransferase neither inosine kinase nor phosphotransferase activity was present. The IMP dehydrogenase differs from the mammalian enzyme in that it does not require for activity and it is more sensitive to inhibition by mycophenolic acid (13). 

Individual enzymes of purine salvage are similar to those of Leishmania. PRTase activities were found for adenine, hypoxanthine, and guanine in the three forms (43). As in Leishmania, there is also a separate xanthine PRTase. Nucleoside kinase activities were found for adenosine, inosine, and guanosine (43), nucleoside hydrolase activities for inosine and guanosine and a nucleoside phosphorylase activity for adenosine. There are both nucleoside hydrolase and phosphorylase activities in epimastigotes (44,45). The adenylosuccinate synthetase and adenylosuccinate lyase are essentially identical to those found in L. donovani (46). 

T.b. gambiense bloodstream forms have APRTase, HGPRTase, adenosine kinase and adenylosuccinate synthetase but lack adenosine deaminase. Two phosphorylase activities have been described for the bloodstream forms of T.b. brucei (42,50). One catalyzes the reversible phosphorolysis of adenosine, inosine and guanosine and the other is specific for adenosine and methylthioadenosine. Guanine deaminase is present whereas both adenosine and adenine deaminase are absent (8). Similar results have been reported for T. congolense (51). T. vivax is unique among the other trypanosomes in that it has an adenine deaminase (51). 

Adenosine and AMP can be deaminated by adenosine deaminase and AMP deaminase, respectively, to form inosine and IMP (see Fig. 41.10). Adenosine is also the only nucleoside to be directly phosphorylated to a nucleotide by adenosine kinase. Guanosine and inosine must be converted to free bases by purine nucleoside phosphorylase before they can be converted to nucleotides by HGPRT. 

Poly(7-deazaguanylic acid) has been prepared by polymerization of 7-deaza-guanosine 5 -diphosphate using polynucleotide phosphorylase from Micrococcus luteus, and its ability to form duplexes with complementary polynucleotides compared with that of poly(G). It is cleaved by ribonuclease T1 and also by nuclease SI, showing that unlike poly(G), it forms no selfstructure in aqueous solution. ... 

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