RECYCLING OF ALPHA-D-RIBOSE 1-PHOSPHATE FOR NUCLEOSIDE INTERCONVERSION

Mobilization of the ribose moiety and of the amino group of guanosine may be realized in rat liver extract by the concerted action of purine nucleoside phosphorylase and guanase. Ribose I-phosphate formed from guanosine through the action of purine nucleoside phosphorylase acts a s ribose donor in the synthesis of xanthosine catalyzed by the same en zyme. The presence of guanase, which irreversibly converts guanine to xanthine, affects the overall process of guanosine transformation. As a result of this purine pathway, guanosine is converted into xanthosin e, thus overcoming the lack of guanosine deaminase in mammals. Further more, in rat liver extract the activated ribose moiety stemming from t he catabolism of purine nucleosides can be transferred to uracil and, in the presence of ATP, used for the synthesis of pyrimidine nucleotid es; therefore, purine nucleosides can act as ribose donors for the sal vage of pyrimidine bases.