Leupeptin

Leupeptin is a protease inhibitor that also acts as an inhibitor of calpain.

It is often used during in vitro experiments when a specific enzymatic reaction is being studied. When cells are lysed for these studies proteases that are usually contained with vacuoles called lysosomes are released. These proteases if freely present in the lysate would destroy any products from the reaction being studied and make the experiment uninterpretable. For example, it could be used in a calpain extraction to keep it from being hydrolyzed by specific proteases.