E-64,66701-25-5,IC-0209597

A new class of compounds that show promise of acting as class-specific inhibitors for the cysteine proteinases are the L-trans-epoxysuccinylpeptides related to the compound E-64 [L-trans-epoxysuccinyl-L-leucylamido(4-guanidino)butanel , isolated from cultures of Aspergillus. E-64 was shown to inhibit papain, ficin and the fruit and stem bromelains, with disappearance of the thiol group of papain1.
E-64 has been reported to inhibit two other mammalian cysteine proteinases: cathepsin L3 and a proteinase from human breast-tumour tissue4 and the calcium-dependent proteinase, calpain, from chicken muscle5. All of these characteristics suggested that E-64 might be a valuable inhibitor for the study of cysteine proteinases.
Lineweaver-Burk plots of inhibition data show that the action of E-64 was not competitive with substrate1 . Moreover, the optical isomerism of the epoxysuccinyl moiety seemed to have no effect on the activity of E-64 as an inhibitor of papain6, 7 .If E-64 were indeed acting by covalent reaction at the active site, its rate of reaction would be decreased by the presence of leupeptin, a tight-binding reversible inhibitor8.
E-64 inhibits only cysteine proteinases. Papain showed a particularly high reactivity with E-64, and good rates were also obtained with the other plant enzymes and the lysosomal cysteine proteinases. There is structural evidence that these enzymes form a homologous group9, and they resemble each other in having Mr about 25 000, no (detected) zymogens and no distinct requirement for calcium. Chicken skeletal-muscle calpain is reported to be inhibited by E-64, but the rate constant has not been determined5.
The most obvious practical application of E-64 is in the active-site titration of the papain-related cysteine proteinases. Active-site titration as a method of determining enzyme concentration has the advantage over rate assays of being insensitive to reaction conditions, and giving a result in active-site molarity10 (Bender et al., 1966).