Uridine phosphorolysis

The catabolism of uridine and cytidine by this route has value to the cell in that the ribose 1-phosphate produced may be degraded in energy-yielding reactions of the glycolytic pathway (see Chapter 6). Because of the ready reversibility of uridine phosphorolysis, this enzyme may contribute to the anabolism of uracil. 

The reversibility of uridine phosphorolysis suggests that uridine phosphorylase activity of the cell may be able to operate in the direction of synthesis, utilizing endogenous ribose 1-phosphate, and thereby contribute to the anabolism of uracil. Thus, uridine phosphorylase and uridine kinase acting in sequence would elevate uracil to the nucleotide level. 

Phosphorolysis of ribonucleic acid with polynucleotide phosphorylase gives a mixture of the diphosphates of the four common nucleosides, which are transformed into triphosphates with enolpyruvate phosphate and pyruvate kinase. This mixture may be used as such as a source of uridine triphosphate in the preparation of the nucleotide-sugar uridine 5 -(a-D-glucopy-ranosyl diphosphate) ( uridine-diphosphate-glucose, UDP-Glc), or as a... 

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. 

Phosphorolysis of ribosylpurines, 2-deoxy-D-ribosyl -purines, uridine, and thymidine has been detected in a number of bacteria. ... 

Uridine phosphorylase participates in ribosyl transfer reactions which take place by the following mechanism. Ribose 1-phosphate produced by phosphorolysis of one nucleoside may serve as the ribose source in the phosphoiylase-catalyzed synthesis of a different nucleoside ... 

Deoxyribonucleoside phosphorolysis is catalyzed by purine nucleoside phosphorylase, thymidine phosphorylase, and to some extent by uridine phosphorylase the specificity of animal uridine phosphorylases differs with the cells of origin (see Chapter 12). 

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