In enzymology, a purine-nucleoside phosphorylase (EC 2.4.2.1) is an enzyme that catalyzes the chemical reaction:purine nucleoside + phosphate purine + alpha-D-ribose 1-phosphate. Thus, the two substrates of this enzyme are purine nucleoside and phosphate, whereas its two products are purine and alpha-D-ribose 1-phosphate. This enzyme belongs to the family of glycosyltransferases, specifically the pentosyltransferases. This enzyme participates in 3 metabolic pathways:purine metabolism, pyrimidine metabolism, and nicotinate and nicotinamide metabolism.http://www.creative-enzymes.com/product/Native-Microorganisms-Nucleoside-Phosphorylase_1329.html