Removal of Endotoxin from Recombinant Protein Preparations

Objectives: To develop an effective method to remove endotoxin from large scale E. coli recombinant protein purifications. Design and Methods: Triton X-114 phase separation, affinity chromatography utilizing immobilized polymyxin B or immobilized histidine, were used to remove endotoxin from purified preparations of recombinant CK-BB, CK-MB, CK-MM, myoglobin, and cardiac troponin I. Endotoxin levels were measured by a Limulus Amebocyte Lysate gel-clot assay. The immunoactivity of these protein preparations was determined by BIAcore™ analysis using a panel of in-house generated monoclonal antibodies and by a Stratus® Fluorometric Analyzer. In the case of troponin I, the BIAcore™ was also utilized to measure troponin C interactions. Results: Phase separation with Triton X-114 was the most effective method in reducing the amount of endotoxin present in the protein preparations compared to either polymyxin B or histidine affinity chromatography. With Triton X-114, the reduction in endotoxin levels was greater than 99% and recovery of the proteins after endotoxin removal was greater than 90%. All three procedures for removing endotoxin had no deleterious effects on the immunoactivity of majority proteins when tested with a panel of monoclonal antibodies. Troponin I also retained its ability to bind to troponin C in the presence of Ca2+. Recombinant CK-BB and CK-MM which were expressed in the soluble fraction of E. coli cell lysates, contained significantly higher endotoxin levels than recombinant CK-MB, myoglobin and cardiac troponin I which were expressed in the form of inclusion bodies. Conclusion: Of the three methods tested, Triton X-114 phase separation was the most effective way of removing endotoxin from recombinant proteins.