According to Chittenden, pepsin is a hydrolytic ferment which is found in the cells of the tubules of the gastric mucous membrane, chiefly near the cardiac portion. It exists in these cells in an antecedent form, or as a granular "proenzyme," which is called pepsinogen or propepsin, and the agent which is believed to convert the propepsin into true pepsin, the active ferment, is hydrochloric acid. This theory assigns a new function to this acid. Lactic and acetic acids derived from food possess the same power in lesser degree. Like the other digestive ferments, pepsin belongs to the class of colloid or non-crystallisable, indiffusible substances.

Pepsin digests coagulated egg albumin even better than fibrin, gluten, casein, myosin, and gelatin, and hence this substance is commonly employed in making quantitative tests of the relative digestive power of different preparations of the ferment. The standard of the United States Pharmacopoeia requires that pepsin shall dissolve three thousand times its own weight of coagulated disintegrated egg albumin. The rapidity of digestion is augmented only up to a certain point by increase in the quantity of pepsin, but beyond this limit it has no accelerating influence - no influence at all, in fact.

Pepsin reaches its maximum activity at a temperature considerably higher than that of the body - namely, 1300 F. Its action is suspended below 400 F., and destroyed between 1600 and 1700 F.

The secretion of both pepsin and the rennet ferment or rennin which is often associated with it seems to rise and fall with that of hydrochloric acid, but this is not always the case.

Rennin has been described on p. 72.