The Collaborative International Dictionary of English v.0.48:

Hypoxanthin \Hy`po*xan"thin\, n. [Pref. hypo- + xanthin.]
   (Physiol. Chem.)
   A crystalline, nitrogenous substance, closely related to
   xanthin and uric acid, widely distributed through the animal
   body, but especially in muscle tissue; -- called also
   sarcin, sarkin.
   [1913 Webster]