Cathepsin S. The enzyme stock solution (in 100 mM MES, pH 6.5, containing 1 mM EDTA, 50 mM L-cysteine, 10 mM DTT, 0.5% Triton X-100, and 30% glycerol) was diluted 1:100 with assay buffer (50 mM sodium phosphate buffer, pH 6.5, 50 mM NaCl, 2 mM EDTA, 0.01% Triton X-100) containing 5 mM DTT, incubated at 37°C for 60 min, and then stored on ice. A 10 mM solution of the substrate Bz-Phe-Val-Arg-pNA was prepared in DMSO. In a cuvette containing 215 µL assay buffer, the chromogenic substrate (1.75 µL) and DMSO and/or inhibitor solution (3.25 µL) were pipetted. Upon addition of CatS (30 µL), the measurement was started and followed at 37 °C for 60 min at 405 nm. The final substrate concentration was 70 µM (= 1.13 × Km), and the final DMSO concentration was 2% (v/v). A Km value of 61.9 ± 11.4 µM for the CatS-catalyzed cleavage of Bz-Phe-Val-Arg-pNA was obtained from quadruplicate measurements with 13 different substrate concentrations. The standard error refers to the non-linear regression.