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, trypsin and plasmin. Little to no inhibition is seen against pepsin, cathepsins A and D and alpha-chymotrypsin. When adjusted for molarity, all three salt forms are equally effective; however, the hydrochloride salt is usually less invasive in biological settings. Leupeptin, because of its aldehyde group, may act as a reducing agent and therefore interfere in protein determinations such as Lowry and, to a lesser extent, Bradford.

• White to off-white powder
• Store at -20 °C; desiccate; protect from light

Leupeptin hydrochloride is used in immunoblotting and acts as a protease inhibitor. Leupeptin gives multiple peaks on HPLC due to equilibria among three forms in solution. Purity determined by HPLC should take into account the three main peaks. Majority of contaminating peptide is racemized leupeptin. The primary mechanism of inactivation is racemization of the L-arginal; the D-arginal form is totally inactive. If the aldehyde is oxidized but retains its L-configuration, the resulting compound does have some inhibitory activity.