Ion-exchange resins greatly facilitate refolding of like-charged proteins at high concentrations

Protein refolding is a crucial step for the production of therapeutic proteins expressed in bacteria as inclusion bodies. In vitro protein refolding is severely impeded by the aggregation of folding intermediates during the folding process, so inhibition of the aggregation is the most effective approach to high-efficiency protein refolding. We have herein found that electrostatic repulsion between like-charged protein and ion exchange gel beads can greatly suppress the aggregation of folding intermediates, leading to the significant increase of native protein recovery. This finding is extensively demonstrated with three different proteins and four kinds of ion-exchange resins when the protein and ion-exchange gel are either positively or negatively charged at the refolding conditions. It is remarkable that the enhancing effect is significant at very high protein concentrations, such as 4 mg/mL lysozyme (positively charged) and 2 mg/mL bovine serum albumin (negatively charged). Moreover, the folding kinetics is not compromised by the presence of the resins, so fast protein refolding is realized at high protein concentrations. It was not realistic by any other approaches. The working mechanism of the like-charged resin is considered due to the charge repulsion that could induce oriented alignment of protein molecules near the charged surface, leading to the inhibition of protein aggregation. The molecular crowding effect induced by the charge repulsion may also contribute to accelerating protein folding. The refolding method with like-charged ion exchangers is simple to perform, and the key material is easy to separate for recycling. Moreover, because ion exchangers can work as adsorbents of oppositely charged impurities, an operation of simultaneous protein refolding and purification is possible. All the characters are desirable for preparative refolding of therapeutic proteins expressed in bacteria as inclusion bodies.