Uridine is a pyrimidine nucleoside composed of an uracil base and a ribose sugar. In cells, uridine is predominantly present in RNA (as uridine monophosphate, UMP). Uridine can be produced through de novo UMP synthesis or salvaged from the blood or intracellular sources like RNA. Under energy stress, uridine is catabolized by uridine phosphorylase (UPP1, UPP2) into uracil and ribose 1-phosphate, where the ribose enters central carbon metabolism to support bioenergetics. Uracil, once considered waste, may have extracellular signaling roles.