Enzymatic synthesis of thiouracil riboside and thiouracil desoxyriboside.

In a survey of various substrates for pyrimidine nucleoside synthesis, it was noted that inorganic orthophosphate was released when 2-thiouracil and desoxyribose-l-phosphate were incubated together with thymidine phosphorylase (1). This finding, taken as a fairly certain indication of nucleoside synthesis, has now been fully substantiated by isolation and characterization of crystalline thiouracil desoxyriboside. Initial experiments were carried out with the more readily available ribose-l-phosphate rather than with desoxyribose-l-phosphate. The ribonucleoside of thiouracil was prepared enzymatically by the Kalckar reaction (2) shown in Reaction 1.