Cysteine proteases are a class of enzymes containing an active-site cysteine residue that is important in protein degradation pathways E-64d, a synthetic analog of E-64 and ethyl ester of E-64c, is an irreversible, membrane-permeable inhibitor of lysosomal and cytosolic cysteine proteases. E-64d inhibits calpain and the cysteine proteases cathepsins F, K, B, H, and L. By disrupting protease activity, E-64d, at concentrations between 20-200 μM, has been shown to arrest human epidermoid carcinoma A431 cells at mitotic metaphase. It also inhibits protease-resistant prion protein accumulation in scrapie-infected neuroblastoma cells with an IC50 value of 0.5 μM.
