A reversible inhibitor of cysteine, serine, and threonine proteases; inhibits cathepsin B (Ki = 6 nM), calpain (Ki = 10 nM), trypsin (Ki = 35 nM), plasmin (Ki = 3.4 μM), and kallikrein (Ki = 19 μM), and has no effect against chymotrypsin, elastase, renin, or pepsin.