*2.11. Effect of Metal Ions on the Activity of Mal-mPEG5000-SPA*

As shown in Table 4, the relative specific activity after addition of Mn2+, K+, Zn2+ and Ca2+ was respectively (143.48 ± 6.25)%, (115.49 ± 4.51)%, (111.28 ± 4.82)% and (102.88 ± 4.32)%, which demonstrated that Mn2+, K+, Zn2+ and Ca2+ had good activation effect. Mg2+ showed a slight inhibitive effect on Mal-mPEG5000-SPA, with relative specific activity standing at (95.25 ± 4.14)%. However, Cu2+, NH4 +, Fe3+, Al3+, Ba2+ and EDTA demonstrated a relatively strong inhibitory effect on Mal-mPEG5000-SPA, with relative specific activity reduced to 55–80%. Meanwhile, under the effect of Cd2+, Hg+ and Ag+, the relative specific activity were all below 30%, among which Hg+ and Ag+ indicated the strongest inhibitory effect on Mal-mPEG5000-SPA. Compared with the effect of metal ions on SPA reported in earlier studies [21], the effect of metal ions on Mal-mPEG5000-SPA and on SPA showed no significant difference (*p* = 0.05).

**Table 4.** Effect of metal ions on the activity of Mal-mPEG5000-SPA.


Significant difference in each column are expressed as different superscript letters (*p* < 0.05).

#### **3. Materials and Methods**

#### *3.1. Materials*

Sweet potato, molar weight 56.043 kDa, harvested in Xinxiang City, Henan Province, China, its cultivar was Shangshu 19. NHS-mPEG5000, NHS-mPEG20000, Ts-mPEG5000, Ts-mPEG10000, Ts-mPEG20000 and Mal-mPEG5000 were from Nanocs (New York, NY, USA; purity ≥ 95%). All other reagents were of analytical reagent grade and used without further purification.
