*2.1. Screening of Modifiers*

The change in SPA specific activity modified by 6 different modifiers was shown in Figure 1. Compared with the untreated SPA, Mal-mPEG5000, NHS-mPEG5000 and Ts-mPEG5000 could significantly increase SPA's specific activity (*p* < 0.05) in the same molar ratio. The specific activity of Mal-mPEG5000-SPA reached the maximum value at (2.081 ± 0.050) × <sup>10</sup><sup>4</sup> U/mg. There was a significant difference between Mal-mPEG5000 and other modifiers (*p* < 0.05). However, there was no significant difference between Ts-mPEG10000, Ts-mPEG20000 and NHS-mPEG20000 (*p* > 0.05). Therefore, Mal-mPEG5000 was selected as the optimal modifier for the next experiments.

**Figure 1.** Effect of 6 types of modifiers on SPA activity. Significant difference in each column are expressed as different superscript letters (*p* < 0.05).
