Crude cell extracts contain many endogeneous enzymes, such as proteases and phosphotases, which can degrade valuable proteins in the extract. The individual components of the protease inhibitor cocktail have specific inhibitory properties. A description of each inhibitor is given below:

Aprotinin: Inhibits serine proteases, such as trypsin, chymotrypsin, plasmin, trypsinogen, urokinase, and kallikrein.

Leupeptin: Inhibits both serine and cysteine proteases, such as calpain, trypsin, papain, and cathepsin B.

Pepstain A: Inhibits acid proteases, such as pepsin, rennin, cathepsin D, chymosin, and protease B.

PMSF: Inhibits serine proteases, such as trypsin and chymotrypsin. Also inhibits cysteine proteases and mammalian acetylcholinesterase.