xanthine \xan"thine\, n. also xanthin \xan"thin\ . gr. xanqo`s
   yellow. physiol. chem.
   a type of purine obtainable as a white microcrystalline
   powder, c5h4o2n4, present in muscle tissue, in the liver,
   spleen, pancreas, and other organs, and also in urine in
   small quantities and some urinary calculi, and in the juices
   of certain plants; -- so called because it leaves a yellow
   residue when evaporated to dryness with nitric acid. it is
   also present in guano. xanthine is closely related to uric
   acid.
   webster 1913 suppl.
see also:
c5h4o2n4