Peptidyl acyloxymethyl ketones, previously established as potent inactivators of the lysosomal cysteine proteinase cathepsin B, were evaluated against smooth-muscle calpain. a member of the family of Ca2+-dependent cysteine proteinases. Only modest rates of time-dependent inhibition could be achieved, even with peptidyl affinity groups optimized for calpain and linked to a carboxylate leaving group of very low pK(a) [2,6-(CF3)2PhCOO-, pK(a) 0.58]. Selective inactivation of cathespin B versus calpain was Consistently observed with this type of inhibitor. Examination of other potential inhibitors revealed a rank order of potency against calpain to be: peptidyl sulphonium methyl ketones > fluoromethyl ketones, diazomethyl ketones much greater than acyloxymethyl ketones, an order which differs sharply from that found for cathespin B.
