0.6 IU/μl. Unit Definition: One unit is defined as the amount of enzyme needed to cleave 100 μg of fusion protein in 16 hours to 90% completion at 4°C in a buffer containing 50 mM Tris-HCl, pH 7.0, 150 mM NaCl, 1 mM EDTA, and 1 mM DTT.