Converts the preformed base xanthine, a product of nucleic acid breakdown, to xanthosine 5'-monophosphate (XMP), so it can be reused for RNA or DNA synthesis. diphosphate + XMP = 5-phospho-alpha-D-ribose 1-diphosphate + xanthine Purine metabolism; XMP biosynthesis via salvage pathway; XMP from xanthine: step 1/1. Homodimer. Belongs to the purine/pyrimidine phosphoribosyltransferase family. Xpt subfamily.