Source : Webster's Revised Unabridged Dictionary (1913)

Xanthine \Xan"thine\, n. Also Xanthin \Xan"thin\ . [Gr. xanqo`s
   yellow.] (Physiol. Chem.)
   A white microcrystalline nitrogenous compound, {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. Xanthine is closely related to
   uric acid.

Source : WordNet®

xanthine
     n : crystalline oxidation product of the metabolism of
         nucleoproteins; precursor of uric acid; found in many
         organs and in urine