Catalyzes the phosphorolysis of diverse nucleosides, yielding D-ribose 1-phosphate and the respective free bases. Can use uridine, adenosine, guanosine, cytidine, thymidine, inosine and xanthosine as substrates. Also catalyzes the reverse reactions. Is not able to produce D-ribose 1-phosphate from D-ribose and phosphate. a purine D-ribonucleoside + phosphate = a purine nucleobase + alpha-D-ribose 1-phosphate adenosine + phosphate = adenine + alpha-D-ribose 1-phosphate cytidine + phosphate = alpha-D-ribose 1-phosphate + cytosine guanosine + phosphate = alpha-D-ribose 1-phosphate + guanine inosine + phosphate = alpha-D-ribose 1-phosphate + hypoxanthine phosphate + thymidine = 2-deoxy-alpha-D-ribose 1-phosphate + thymine phosphate + uridine = alpha-D-ribose 1-phosphate + uracil phosphate + xanthosine = alpha-D-ribose 1-phosphate + xanthine Cells lacking this gene show a consistent change in xanthine level. Belongs to the nucleoside phosphorylase PpnP family.