Uracil phosphorolysis

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. 

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. 

Deoxyuridine and thymidine are substrates for pyrimidine nucleoside phosphorylases, but deoxycytidine (and cytidine) is generally regarded as being inert to phosphorolysis (7) Tarris demonstration of deoxycytidine formation from cytosine in extracts of fish milt is an exception to this generalization (8). Catabolism of C3rtosine nucleosides is initiated by deamination to form uracil nucleosides which can be phosphorolyzed. 

In animal cells engaged in DNA synthesis, deoxyuridine can be incorporated without cleavage into DNA thymidylate, but phosphorolysis also occurs and the uracil so released will not have incorporation into DNA as a specific metabolic fate. Because of deamination, deoxycytidine may be converted to either of the pyrimidine nucleotides of DNA cleavage of the deamination product, deoxyuridine, liberates uracil which may enter the pathways of pyrimidine ribonucleotide metabolism. 

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