Purine nucleoside phosphorylase involved in purine salvage. a purine D-ribonucleoside + phosphate = a purine nucleobase + alpha-D-ribose 1-phosphate Purine metabolism; purine nucleoside salvage. Homotrimer. Although this enzyme belongs to the family of MTA phosphorylases based on sequence homology, it lacks several conserved amino acids in the substrate binding pocket that confer specificity towards MTA. Belongs to the PNP/MTAP phosphorylase family. MTAP subfamily.