E-64 [L-trans-epoxysuccinyl-leucyl-amido(4-guanidino)butane] and related epoxides as inhibitors of cysteine proteinases.

1. E-64 [L-trans-epoxysuccinyl-leucyl-amido(4-guanidino)butane] was found to be an active site directed irreversible inhibitor of all of the cysteine proteinases tested except clostripain, and not to affect other enzymes. Thus it seems to be a powerful selective inhibitor for cysteine proteinases in mammalian systems. 2. The very high rate of inactivation of some cysteine proteinases, including cathepsins B and L, allows the molarity of active enzyme to be determined by stoichiometric titration with E-64. This incidentally calibrates the rate assay of the enzyme in terms of molar concentrations. 3. Measurement of rates of reaction of E-64 and a number of optically active analogues with cathepsins B, H and L has given some insight into structure-activity relationships. Cathepsin H is far less reactive than the other enzymes, and generally cathepsins B and L have reactivities rather similar to each other with the epoxide inhibitors.