E-64, Cysteine Protease Inhibitor: Frequently Asked Questions

E-64 is an irreversible, potent and highly selective inhibitor of cysteine proteases. Does not affect cysteine residues in other enzymes. Acts by forming a thioether bond with thiol of the active cysteine. E-64 will not inhibit serine proteases (except trypsin) inhibits activation-induced programmed cell death and restores defective immune responses in HIV+ donors. Specific active site titrant.

E-64 is a very useful cysteine protease inhibitor for use in in vivo studies because it has a specific inhibition; it is permeable in cells and tissues & has low toxicity. E-64 inhibits calpain, papain, and cathespin B, cathepsin L, bromelain, staphopain, collegenase and ficin.

The compound has been reported to inhibit intracellular Bax protease activity, and reduce oxidative stress, which includes a decrease in MDA levels, ICAM-1 expression and MOP activity. E-64 has also been used to study excystation, (microbial cyst wall breaks down) in Giardia lamblia.

Chemical structure of E-64

Frequently Asked Questions

APPEARANCE: White crystalline powder 

CHEMICAL NAME: trans-Epoxysucciny-L-leucyl-amido(4-guanidino) butane; (L-3-trans-Carboxyoxiran-2-Carbonyl)-L-Leucyl-Admat, L-trans-3-Carboxyoxiran-2-carbonyl-L-leucylagmatine, N-(trans-Epoxysuccinyl)-L-leucine 4-guanidinobutylamide

EXAMPLES OF CYSTEINE PROTEASES: Actinidain, Bromelain, Calpains, Caspases, Cathepsins, Mir1-CP, Papain.

IDENTIFICATION: The compound,E-64 was first isolated and identified from the fungus, Aspergillus japonicus in 1978.[1]



CAS#: 66701-25-5

SOURCE: Natural or synthetic

SOLUBILITY INFORMATION: DMSO (25mg/ml) and H20 (20mg/ml. A 20 mg/ml solution can be prepared in water (heat may be needed).

A suggested water stock solution is a 1 mM aqueous solution).

E-64 is also soluble in DMSO; a 10 mM solution can be prepared in dry DMSO and stored at -20 °C.

If aqueous stock solutions are required for biological experiments, they can best be prepared by diluting the organic solvent into aqueous buffers or isotonic saline. Ensure that the residual amount of organic solvent is insignificant, since organic solvents may have physiological effects at low concentrations. We do not recommend storing the aqueous solution more than one day.

Solutions for injection were prepared by dissolving E-64 in 0.9% sodium chloride or in a minimum amount of saturated sodium bicarbonate followed by dilution with 0.9% sodium chloride (after adjusting the pH to 7.0 with acetic acid.

pH RANGE: Diluted solutions are stable for days at neutral pH. E-64 is stable from pH 2-10


INCOMPATIBILITIES: E-64 is unstable in ammonia or in HCl.

EFFECTIVE CONCENTRATION: The effective concentration for use as a protease inhibitor is 1 to 10 µM.

Storage: Store, as supplied, at -20°C for up to 1 year. Store solutions at -20°C for <3 months.

RTECS#: RR0390000

What is RTECS# and what does it tell us?: (Registry of Toxic effects of Chemical substances) RTECS is a compendium of data extracted from the open scientific literature. The data are recorded in the format developed by the RTECS staff and arranged in alphabetical order by prime chemical name. Six types of toxicity data are included in the file: (1) primary irritation; (2) mutagenic effects; (3) reproductive effects; (4) tumorigenic effects; (5) acute toxicity; and (6) other multiple dose toxicity. Specific numeric toxicity values such as LD50, LC50, TDLo, and TCLo are noted as well as species studied and route of administration used. For each citation, the bibliographic source is listed thereby enabling the user to access the actual studies cited. No attempt has been made to evaluate the studies cited in RTECS. The user has the responsibility of making such assessments

MDL number: MFCD00080261

IC50 in vitro VALUES: Cathepsin K; 1.4nM; Cathepsin S; 4.1nM;  Cathepsin L: 2.5nM2.



K. Hanada, M. Tamai, M. Yamagishi, S. Ohmura, J. Sawada and I. Tanaka, Agric. Biol. Chem., 42, 523 (1978) (Original)

Varughese K, Ahmed F, Carey P, Hasnain S, Huber C, Storer A. (1970). "Crystal structure of a papain-E-64 complex".

Govrin, E., and Levine, A., Purification of active cysteine proteases by affinity chromatography with attached E-64 inhibitor Protein Expr. Purif. 15, 247-250, (1999)

Pickering, W., et al., Stimulation of protein degradation by low pH in L6G8C5 skeletal muscle cells is independent of apoptosis but dependent on differentiation state Nephrol. Dial. Transplant. 18, 1466-74, (2003).

Trümpler, A., et al., Calpain-mediated Degradation of Reversibly Oxidized Protein-tyrosine Phosphatase 1B. FASEB J. 276, 5622-33, (2009).

Shin, Y.P., et al., Antimicrobial Activity Of A Halocidin-derived Peptide Resistant to Attacks by Proteases. Antimicrob. Agents Chemother. 54, 2855-66, (2010) Abstract.

Li, Z., et al., Similarities In The Behavior And Molecular Deficits In The Frontal Cortex Between The Neurotensin Receptor Subtype 1 Knockout Mice And Chronic Phencyclidine-treated Mice: Relevance To Schizophrenia. Neurobiol. Aging 40, 467-77, (2010).

Barrett, A.J., et al. Biochem. J. 201, 189, (1982) Abstract

Beynon, R.J. and Bond, J.S., ed. Proteolytic Enzymes: A Practical Approach New York, NY , (1989), 244

Tamai, M., et al. 1987. Chem. Pharm. Bull. 35: 1098-1104.

Wood, D.E., et al. 1998. Oncogene. 17: 1069-1078.

Szpaderska, A.M. and Frankfater, A. 2001. Cancer Res. 61: 3493-3500.

Chatterjee, P.K., et al. 2005. Biochem. Pharmacol. 69: 1121-1131.

Hussein, E.M., et al. 2009. J Egypt Soc Parasitol. 39: 111-119.

Byrne, D.P., et al. 2010. Biochem. J. 425: 257-264.


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