Purine Nucleoside Phosphorylase: Kinetics, Mechanism, and Specificity

Ribosyl exchange reactions between purines and purine nucleosides catalyzed by crystalline purine nucleoside phosphorylase from calf spleen were markedly stimulated by inorganic phosphate. Initial velocity and product inhibition analyses were performed for the enzymic synthesis of inosine from ribose 1-phosphate and hypoxanthine. An ordered sequential mechanism involving the isomerization of time free enzyme and the initial combination of the purine base with one of the forms of time free enzyme is most consistent with these results. The specificity of the enzyme toward the purine base in the synthesis of ribonucleosides was investigated. In order of decreasing effectiveness, ribosyl acceptors were guanine, hypoxanthine, xanthine, 6-mercaptopurine, and allopurinol [4-hydroxypyrazolo(3,4-d)pyrimidine]. Oxoallopurinol [4,6-dihydroxypyrazolo-(3,4-d)pyrimidine] was a very poor substrate. Adenine, azathioprine [6-(1-mlethyl-4-nitro-5-imidazolyl)thiopurine], and uracil did not produce detectable amounts of ribonucleoside. Guanosine, deoxyguanosine, and inosine were shown to be effective pentosyl donors in exchange reactions.