Zymogens, or proenzymes, are enzymes secreted in inactive form. Under certain conditions, a zymogen shifts to the active form of the enzyme. Zymogen secretions in general occur due to the enzyme activity can harm the secretory tissue.
For example, the pepsinogen secreted by the stomach becomes active under acid pH turning into the enzyme pepsin. Other well-known zymogens are trypsinogen and chymotrypsinogen. Enzymes are secreted by the exocrine pancreas and correspondingly trypsin and chymotrypsin.