Recombinant Protective Antigen (PA) for use in the enzyme-linked immunosorbent assay (ELISA) and the cytokine assays was purified as follows. E. coli Top 10 F0 cells, harboring the plasmids prep4 and pQE30 (both from Qiagen), encoding PA83, were grown to an optical density at 600 nm of 0.6. Transcription was then induced with 1 mM isopropyl-b-d-thiogalactopyranoside. Bacteria were harvested 3 h later, lysed with 6 M guanidine in the presence of proteinase inhibitors (Sigma, Taufkirchen, Germany) and purified on a Ni2+-loaded HiTrap chelating column (Amersham Biosciences, Freiburg, Germany). After dialysis against 20 mM Tris, 5 mM EDTA (pH8.9), the protein fraction was further purified using an anion exchange column (HiTrap Q HP, Amersham), and finally dialyzed against phosphatebuffered saline (PBS). We have it in a concetration (PA) 1.269mg/ml) for i-ELISA.
In principle I dont see any problem for conjugation, but you should look for the datasheet of the particular beads you want to use to see if the buffer and concentration are compatible. And it would be good to have evidences that your protein is correctly folded, like recognition by antibodies against the native protein.
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