Preparation of carboxylated magnetic particles for the eYcient immobilization of C-terminally lysine-tagged Bacillus stearothermophilus aminopeptidase II

This article reports the synthesis and use of surface-modiWed iron oxide particles for the simultaneous puriWcation and immobilization of Bacillus stearothermophilus aminopeptidase II (BsAPII) tagged C-terminally with either tri- or nona-lysines (BsAPII-Lys_3/9). The carboxylated magnetic particles were prepared by the simple co-precipitation of Fe³⁺/ Fe²⁺ in aqueous medium and then subsequently modiWed with adipic acid. Transmission electron microscopy (TEM) micrographs showed that the carboxylated magnetic particles remained discrete and had no signiWcant change in size after binding BsAPIIs. Wild-type enzyme and BsAPII-Lys₃ could be puriWed to near homogeneity by the carboxylated magnetic particles, but it was not easy to elute the adsorbed BsAPII-Lys₉ from the matrix. Free BsAPII, BsAPII-Lys3, and BsAPII-Lys₉ were active in the temperature range 50-70°C and all had an optimum of 50°C, whereas the optimum temperature and thermal stability of BsAPII-Lys₃ and BsAPII-Lys ₉ were improved as a result of immobilization. The immobilized BsAPII-Lys₉ could be recycled ten times without a signiWcant loss of the enzyme activity and had a better stability during storage than BsAPII. Owing to its high eYciency and cost-eVectiveness, this magnetic adsorbent may be used as a novel puriWcation-immobilization system for the positively charged enzymes.