TCA–NSI
This was conducted according to the method by Chen et al. (2021).
Briefly, TCA was added into samples to a final concentration of 15%.
The mixtures were mixed well, then stood for 1 h at room temperature.
Subsequently, the samples were centrifuged (7000g , 15 min) to
obtain transparent supernatants, whose nitrogen contents were determined
by the micro-Kjeldahl method. TCA–NSI was calculated as nitrogen
content in supernatant/nitrogen content in original mixture *100%,
which could be used to quantitatively explain the combined activity of
endopeptidases and exopeptidases.