Leupeptin is a modified tripeptide reversible protease inhibitor of trypsin-like proteases and some cysteine proteases including endoproteinase Lys-C, kallikrein, papain, thrombin, cathepsin B, cathepsins H and L, trypsin, calpain, trypsin8 and plasmin.

• Soluble in water (50 mg/ml)
• White powder
• Effective concentration: 10 to 100 μM

It gives multiple peaks on HPLC due to equilibria among three forms in solution. Purity determined using three main peaks. Majority of contaminating peptide is racemised leupeptin.