Nucleoside

Uridine is a typical example. A review of its biochemistry describes uridine as a pyrimidine nucleoside that takes part in the synthesis of RNA, glycogen and cell membranes [2]. In humans, uridine is present in plasma in considerably higher quantities than other purine and pyrimidine nucleosides [2].

Food supplies nucleosides indirectly. A review of milk constituents explains that the main dietary sources are nucleoproteins and nucleic acids, which digestion converts into nucleosides and nucleobases, the forms preferred for absorption in the intestine [3].

Inside the body, cells take up uridine through the nucleoside transport pathway [2]. Uptake into the brain begins at nucleoside transporters in the blood-brain barrier; the high-affinity transporter for uridine has been found in blood-brain barrier cells of rats [4]. Once inside, uridine contributes to the brain's synthesis of phosphatidylcholine, through a route the Nucleotide entry describes (a mechanism, not a measured outcome) [4].

The word tells you about structure, so it helps to tell apart related compounds on a label or in a paper. A nucleoside has a base and a sugar; a nucleotide has the same parts plus phosphate [1].

Blood levels matter here too. A review of uridine says its biological actions may be related to its plasma concentration, and it lists factors that raise or lower that concentration, from ATP consumption to uptake by cells through the nucleoside transport pathway [2].

• Nucleotide: a nucleoside with phosphate attached [1].
• Blood-brain barrier: where nucleoside transporters begin uptake into the brain [4].