Converts the preformed base xanthine, a product of nucleic acid breakdown, to xanthosine 5'-monophosphate (XMP), so that it can be reused for RNA or DNA synthesis. diphosphate + XMP = 5-phospho-alpha-D-ribose 1-diphosphate + xanthine Optimum pH is 6-9. Purine metabolism; XMP biosynthesis via salvage pathway; XMP from xanthine: step 1/1. Homodimer. Repressed during growth on purines as a nitrogen source. Highly specific for xanthine, with a strong discrimination against hypoxanthine and guanine as substrates. Belongs to the purine/pyrimidine phosphoribosyltransferase family. Xpt subfamily.