Dilute recombinant human Meprin alpha Subunit (R&D Systems, Catalog # 3220-ZN) to 0.1 mg/mL in TCNB buffer [50 mM Tris, 150 mM NaCl, 10 mM CaCl2 (Sigma, Catalog # C-5080), 0.05% Brij® 35, pH 7.5] containing 0.1 µg/mL Trypsin. Mix well by gently vortexing.
Incubate the mixture at 37° C for 3 hours in a heat block covered with foil. Stop trypsin activity by adding 1 mM AEBSF (R&D Systems, Catalog # EI001).