Catalyzes the reversible phosphorylytic cleavage of uridine and deoxyuridine to uracil and ribose- or deoxyribose-1-phosphate (PubMed:14715930). The produced molecules are then utilized as carbon and energy sources or in the rescue of pyrimidine bases for nucleotide synthesis (Probable). phosphate + uridine = alpha-D-ribose 1-phosphate + uracil A conditional disulfide bridge can form within the protein that dislocates a critical phosphate-coordinating arginine Arg-100 away from the active site, disabling the enzyme. Pyrimidine metabolism; UMP biosynthesis via salvage pathway; uracil from uridine (phosphorylase route): step 1/1. Homodimer. Liver specific. Directly up-regulated in liver by HNF-4-alpha (HNF4A) binding to the promoter. May also be indirectly regulated by signaling via various orphan nuclear receptors. Strongly up-regulated by a liver X receptor (LXR) agonist. Slightly up-regulated by a pregnane X receptor (PXR) agonist. Strongly repressed by a peroxisome proliferator-activated receptor alpha (PPAR-alpha) agonist. Slightly repressed by a farnesoid X receptor (FXR) agonist. Shows a diurnal expression pattern with peak levels 12 hours after light onset and lowest levels 0 hours after light onset. Belongs to the PNP/UDP phosphorylase family.