Recovery rate of NK by purification
The recovery rate of NK obtained by ammonium sulfate precipitation was 89.1% (Table 2), which was consistent with a study by Garg [16]. The high recovery rate and simple operation showed that this method could be used to purify NK on a large scale.
The purification was followed by Ni–NTA affinity chromatography, and the total recovery rate was 65.2% (Table 2). The high imidazole concentration in wash buffer B led to a high loss of NK when samples were purified using Ni–NTA affinity chromatography. Accordingly, the imidazole concentrations were adjusted to zero in wash buffer A and incubated supernatant, and to 10 mmol L-1 in wash buffer B; thus, a recovery rate of 88% was obtained in this step. Different imidazole concentrations in elution buffer C were also investigated, but there were no significant differences in the range of 100 mmol L-1 to 500 mmol L-1. According to the NK expression level and the recovery rate, the process developed here may be applied for large scale production of NK.