Guanosine phosphorolysis

Phosphorolysis of the glycosyl esters of sugar nucleotides leads to the corresponding nucleoside 5 -pyrophosphate and glycosyl phosphate. A wheat-germ enzyme specific for adenosine 5 -(a-D-glucopy-ranosyl pyrophosphate),462 and a yeast enzyme specific for guanosine 5 -(a-D-mannopyranosyl pyrophosphate),463,464 are known. 

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. 

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. 

Bovine PNP is involved in the mammalian purine salvage pathway, catalyzing phosphorolysis of inosine, guanosine or their 2 -deoxy analogues (equation 9). It is a trimeric enzyme with three identical subunits. It displays third-of-sites activity, with only one subunit active at any time. KIEs were measured for phosphorolysis of inosine by bovine spleen PNP. The 1 - H KIE = 1.047 0.002 for the overall reaction was equal to the previously measured binding KIE = 1.047 0.02 for formation of the Michaelis complex, PNP pho-sphatednosine. This, along with primary KIE = 1.000 0.001 for the... 

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). 

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