Uridine, also known as beta-uridine or 1-beta-D-ribofuranosylpyrimidine-2,4(1H,3H)-dione, is a member of the class of compounds known as pyrimidine nucleosides. Pyrimidine nucleosides are compounds comprising a pyrimidine base attached to a ribosyl or deoxyribosyl moiety. More specifically, uridine is a nucleoside consisting of uracil and D-ribose and a component of RNA. Uridine is soluble (in water) and a very weakly acidic compound (based on its pKa). Uridine can be synthesized from uracil. It is one of the five standard nucleosides which make up nucleic acids, the others being adenosine, thymidine, cytidine and guanosine. The five nucleosides are commonly abbreviated to their one-letter codes U, A, T, C and G respectively. Uridine is also a parent compound for other transformation products, including but not limited to, nikkomycin Z, 3'-(enolpyruvyl)uridine 5'-monophosphate, and 5-aminomethyl-2-thiouridine. Uridine can be found in most biofluids, including urine, breast milk, cerebrospinal fluid (CSF), and blood. Within the cell, uridine is primarily located in the mitochondria, in the nucleus and the lysosome. It can also be found in the extracellular space. As an essential nucleoside, uridine exists in all living species, ranging from bacteria to humans. In humans, uridine is involved in several metabolic disorders, some of which include dhydropyrimidinase deficiency,  MNGIE (mitochondrial neurogastrointestinal encephalopathy), and beta-ureidopropionase deficiency. Moreover, uridine is found to be associated with Lesch-Nyhan syndrome, which is an inborn error of metabolism. Uridine is a nucleoside consisting of uracil and D-ribose and a component of RNA. Uridine plays a role in the glycolysis pathway of galactose. In humans there is no catabolic process to metabolize galactose. Therefore, galactose is converted to glucose and metabolized via the normal glucose metabolism pathways. More specifically, consumed galactose is converted into galactose 1-phosphate (Gal-1-P). This molecule is a substrate for the enzyme galactose-1-phosphate uridyl transferase which transfers a UDP molecule to the galactose molecule. The end result is UDP-galactose and glucose-1-phosphate. This process is continued to allow the proper glycolysis of galactose. Uridine is found in many foods (anything containing RNA) but is destroyed in the liver and gastrointestinal tract, and so no food, when consumed, has ever been reliably shown to elevate blood uridine levels. On the other hand, consumption of RNA-rich foods may lead to high levels of purines (adenine and guanosine) in blood. High levels of purines are known to increase uric acid production and may aggravate or lead to conditions such as gout.