The Collaborative International Dictionary of English v.0.48:

Protamin \Pro"ta*min\, n. [Gr. prw^tos first.] (Physiol. Chem.)
   An amorphous nitrogenous substance found in the spermatic
   fluid of salmon. It is soluble in water, which an alkaline
   reaction, and unites with acids and metallic bases.
   [1913 Webster]