Nucleoside phosphorylase (NP), also designated inosine phosphorylase, forms a homotrimer. It belongs to the PNP/MTAP phosphorylase family of proteins. Nucleoside phosphorylase is an enzyme which cleaves a nucleoside by phosphorylating the ribose to produce a nucleobase and ribose 1 phosphate. It is one enzyme of the nucleotide salvage pathways. These pathways allow the cell to produce nucleotide monophosphates when the de novo synthesis pathway has been interrupted or is non-existent. Often the de novo pathway is interrupted as a result of chemotherapy drugs such as methotrexate or aminopterin.
