Inosine, also known as hypoxanthosine or inotin, belongs to the class of organic compounds known as purine nucleosides. Purine nucleosides are compounds comprising a purine base attached to a ribosyl or deoxyribosyl moiety. Inosine is formed when hypoxanthine is attached to a ribose ring a beta-N9-glycosidic bond. Inosine is an intermediate in the degradation of purines and purine nucleosides to uric acid. Inosine is also an intermediate in the purine salvage pathway. Inosine occurs in the anticodon of certain transfer RNA molecules and is essential for proper translation of the genetic code in wobble base pairs. Inosine exists in all living species, ranging from bacteria to plants to humans. Inosine participates in a number of enzymatic reactions. In particular, inosine can be biosynthesized from inosinic acid through its interaction with the enzyme known as cytosolic purine 5'-nucleotidase. In addition, inosine can be converted into hypoxanthine and ribose 1-phosphate through its interaction with the enzyme known as purine nucleoside phosphorylase. Altered levels of inosine have also been associated with purine nucleoside phosphorylase deficiency and xanthinuria type I, both of which are inborn errors of metabolism. Animal studies have suggested that inosine has neuroprotective properties. It has been proposed as a potential treatment for spinal cord injury (PMID: 16317421) and for administration after stroke, as inosine appears to induce axonal rewiring (PMID: 12084941). After ingestion, inosine is metabolized into uric acid, which has been found to be a natural antioxidant and peroxynitrite scavenger. As such, inosine may have potential benefits to patients with multiple sclerosis and Parkinson‚Äôs disease (PMID: 19425822). Inosine can also be produced by gut bacteria and appears to have a number of beneficial effects.  Inosine, has been shown to activate peroxisome proliferator-activated receptor (PPAR)-gamma signaling in human colon epithelial cells. Furthermore, exogenous treatment of inosine has been found to protect against DSS-induced colitis in rodents by improving adenosine 2A receptor (A2AR)/PPAR-gamma-dependent mucosal barrier functions (PMID: 33820558). Microbiome-derived inosine has also been shown to modulate the response to checkpoint inhibitor immunotherapy in cancer models. In particular, decreased gut barrier function induced by immunotherapy increases systemic translocation of bacterially derived inosine and activates antitumor T cells. The effect of inosine is dependent on T cell expression of the adenosine A2A receptor and requires co-stimulation. Inosine appears to have other roles in non-mammalian system. For instance, it has been found to be an important feed stimulant by itself or in combination with certain amino acids in some species of farmed fish. For example, inosine and inosine-5-monophosphate have been reported as specific feeding stimulants for turbot fry, (Scophthalmus maximus) and Japanese amberjack.