Tricine–SDS–PAGE
This was conducted according to the method by Ying et al. (2015). Briefly, the protein concentrations of all samples above were diluted to 2 mg/mL, and mixed with Tricine–SDS–PAGE sample buffer to make the SDS/protein ratio of 1.52/1. These mixtures were vortexed for 2 min at room temperature. β-mercaptoethanol was added to a final concentration of 2% (v/v), and heated in a boiling water bath for 3 min. Each sample (10 μL) was loaded into a sample well by a microsyringe, and electrophoresed at constant voltage of 30 mV until all samples entered into the stacking gel and then at constant voltage of 100 mV until end. The gel was stained with Coomassie Blue G-250. By analyzing the band intensities of interested proteins (compared to control), the endopeptidase activity could be explained.