E-64

E-64 is a cysteine protease inhibitor that was isolated from the mold Aspergillus japonicus TPR-64. E-64 is also known as N-[N-(L-3-trans-carboxyoxiran-2-carbonyl)-L-leucyl]-agmatine. E-64 effectively inhibits various cysteine proteases, in particular:

• cathepsin K
• cathepsin L
• cathepsin S

E-64 also acts against other enzymes, such as:

• calpain
• cathepsin B
• cathepsin H
• papain

E-64 is an effective ligand for affinity purification of cysteine proteases. When coupled to a thiolated affinity matrix, binding is no longer irreversible, but specificity is retained.

E-64 is an irreversible, potent, and highly selective cysteine protease inhibitor. E-64 does not react with the functional thiol group of non-protease enzymes, such as L-lactate dehydrogenase or creatine kinase. E-64 will not inhibit serine proteases (except trypsin) like other cysteine protease inhibitors, leupeptin and antipain. The trans-epoxysuccinyl group (active moiety) of E-64 irreversibly binds to an active thiol group in many cysteine proteases, such as papain, actinidase, and cathepsins B, H, and L to form a thioether linkage. 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 and has low toxicity.

E-64 is soluble in water. A 20 mg/ml solution can be prepared in water (heat may be needed). A suggested stock solution is a 1 mM aqueous solution. The effective concentration for use as a protease inhibitor is 1 to10 μM. Aqueous stock solutions are stable for months at -20 °C. Diluted solutions are stable for days at neutral pH. E-64 is stable from pH 2-10, but is unstable in ammonia or in HCl. E-64 is also soluble in DMSO, a 10 mM solution can be prepared in dry DMSO and stored at -20 °C. Subsequent dilutions were in culture medium. 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).