Pepsin (pepsinum) is an enzyme usually obtained from the fresh mucous membrane of the hog's stomach. It is almost entirely soluble in 50 parts of water, and more so in water acidulated with hydrochloric acid. It acts in a weakly acid medium to change the insoluble proteins of the food into soluble protein. It is destroyed by 0.01 per cent. sodium hydroxide (Sollmann), and it is inhibited by strong acid, human pepsin, for example ceasing to act when the hydrochloric acid reaches 0.3 per cent. By the U. S. P. test it must be able to change 3000 times its weight of coagulated egg-albumin into soluble protein. In other words, one grain of pepsin can digest at least 6 1/4 ounces of coagulated egg-albumin. Dr. Gies has told me of a specimen in existence 200 times as powerful as this. The U. S. P. test calls for digestion at 125.6° F. (520 C.) for two and one-half hours in water containing one part of absolute hydrochloric acid in 3000.

Pepsin is, therefore, a highly powerful substance; and it would be a very important therapeutic agent were it not for the fact that in almost all classes of digestive disturbances it is a superfluous remedy. For by extensive tests with human gastric contents it has been found that, except in the not very numerous cases of achylia gastrica with atrophy of the gastric mucous membrane, the stomach rarely fails to secrete its specific ferments.

Hence its only use as a digestive agent is in atrophic cases, and in these it is not always efficient. (See Pancreatin.) It may be given in capsules, 5 grains (0.3 gm.) at the beginning of a meal and 5 grains at the end, with hydrochloric acid in proper dilution. Pepsin regularly contains some rennin; its solutions, therefore, will coagulate milk.