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 a moderately basic compound (based on its pKa). Inosine exists in all living species, ranging from bacteria to humans. Within humans, inosine participates in a number of enzymatic reactions. In particular, inosine can be biosynthesized from inosinic acid through its interaction with the enzyme cytosolic purine 5'-nucleotidase. In addition, inosine can be converted into hypoxanthine and ribose 1-phosphate through its interaction with the enzyme purine nucleoside phosphorylase. Inosine, with regard to humans, has been found to be associated with several diseases such as ulcerative colitis, degenerative disc disease, kidney disease, and gout. Inosine is a purine nucleoside that has hypoxanthine linked by the N9 nitrogen to the C1 carbon of ribose. It is an intermediate in the degradation of purines and purine nucleosides to uric acid and in pathways of purine salvage. It also occurs in the anticodon of certain transfer RNA molecules (Dorland, 28th ed). Inosine is found to be associated with purine nucleoside phosphorylase deficiency and xanthinuria type I, which are inborn errors of metabolism.