Pepsin \Pep"sin\ (p[e^]p"s[i^]n), n. [Gr. pe`psis a
         cooking,
   digesting, digestion, fr. pe`ptein, pe`ssein,
         to cook,
   digest: cf. F. pepsine. Cf. Dyspepsia.] (Physiol.
         Chem.)
   A proteolytic enzyme (MW 34,500) contained in the
         secretory
   glands of the stomach. In the gastric juice it
         is united with
   dilute hydrochloric acid (0.2 per cent,
         approximately) and
   the two together constitute the
         active portion of the
   digestive fluid. It degrades
         proteins to proteoses and
   peptides, and is notable for
         having a very low pH optimum for
   its activity. It is the
         active agent in the gastric juice of
   all animals.
  
         [1913 Webster + PJC]

   Note: As prepared from the
         glandular layer of pigs' or
         calves' stomachs it constitutes an important article of
         pharmacy.
         [1913 Webster]