In enzymology, a pyrimidine-nucleoside phosphorylase is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are pyrimidine nucleoside and phosphate, whereas its two products are pyrimidine base and alpha-D-ribose 1-phosphate. This enzyme belongs to the family of glycosyltransferases, specifically the pentosyltransferases. The systematic name of this enzyme class is pyrimidine-nucleoside:phosphate alpha-D-ribosyltransferase. This enzyme is also called Py-NPase. This enzyme participates in pyrimidine metabolism.

Structural studies

As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and.