Purification and Comparative Properties of a Pyrimidine Nucleoside Phosphorylase from Bacillus stearothermophilus

The pyrimidine-nucleoside:orthophosphate ribosyltransferase (EC 2.4.2.2), or pyrimidine nucleoside phosphorylase, from Bacillus stearothermophilus has been purified and compared with similar enzymes from Bacillus subtilis and Escherichia coli. The enzyme is stable at 60°, appears to be induced by either uridine or thymidine, and has a molecular weight of approximately 78,000. Unlike nucleoside phosphorylases from most other sources, the thermophile enzyme catalyzes the phosphorolysis of both thymidine (apparent Km = 3.8 x 10-4 m) and uridine (apparent Km = 2.5 x 10-4 m) and, in the synthetic direction, does not distinguish between ribose 1-phosphate and deoxyribose 1-phosphate. It differs from E. coli and B. subtilis nucleoside phosphorylases in many properties including molecular weight, substrate specificity, thermostability, and induction properties.