Peptidyl and azapeptidyl methylketones as substrate analog inhibitors of papain and cathepsin B

Peptidyl methylketones containing Phe, Tyr, Tyr(I) Tyr(I-2), Leu and Ile in P-2 were synthesized and tested as substrate analog reversible inhibitors of papain and bovine spleen cathepsin B. The most effective cathepsin B inhibitor contained Tyr(I-2) and displayed an inhibition constant of 4.7 mu M at pH 6.8 and 25 degrees C, while Leu or lie gave practically inert analogs. Replacement of the amino acids in P-2 with the analogous alpha-azaamino acids, as well as the glycine in P-1 with alpha-azaglycine, led to complete loss of inhibiting activity. Introducing alkoxy substituents at the methyl adjacent to the ketone group generally resulted in more effective inhibitors, with inhibition constants in the micromolar range for both papain and cathepsin B.