Cathepsin B. The enzyme stock solution (in 20 mM sodium acetate buffer pH 5.5, containing 1 mM EDTA) was diluted 1:500 with assay buffer (100 mM sodium phosphate buffer pH 6.0, 100 mM NaCl, 5 mM EDTA, and 0.01% (w/v) Brij 35) containing 5 mM DTT, incubated at 37 °C for 30 min, and then stored on ice. A 100 mM solution of the substrate Z-Arg-Arg-pNA was prepared in DMSO. Into a cuvette containing 950 μL assay buffer, the chromogenic substrate (5 μL) and DMSO and/or inhibitor solution (15 μL) was pipetted. Upon addition of CatB (30 μL), the measurement was started and followed at 37 °C for 60 min at 405 nm. The final substrate concentration was 500 μM (= 0.45 × Km), and the final DMSO concentration was 2% (v/v).