Peptidylmethylsulphonium salts incorporating consecutive basic residues at the C-terminus of the peptidyl portion such as -Arg-Arg-, -Arg-Lys-, -Lys-Lys- and -Lys-Arg- were synthesized and examined as proteinase inhibitors. Serine proteinases with a specificity directed towards hydrolysis at cationic residues were found to be unaffected by these derivatives. On the other hand, cysteine proteinases, cathepsin B and, in particular, clostripain were readily inactivated by affinity labelling. The reagents thus are of promise for the study of prohormone processing promoted by cysteine proteinases.
