Researchers at the Veterans Affairs Medical Center in Washington, DC compared two commercial preparations of the same protein. Gels and antibody blots showed identical Hsp70 content in both.
The cleaner preparation carried 1.4 picograms of endotoxin per microgram and triggered no TNF-alpha release from mouse macrophage cells, even at 5 micrograms per milliliter. The other carried 0.2 nanograms per microgram, about 140 times more, and matched an equal dose of pure endotoxin.
Stripping the endotoxin out with polymyxin B erased the signal, and adding the same amount to the clean protein recreated it. The two preparations matched on gels, blots and an enzyme test. Only the measured endotoxin number told them apart.
Read the original study
Gao and Tsan, Journal of Biological Chemistry, 2003 · doi.org/10.1074/jbc.M208742200
The signal belonged to the contaminant.
Source
- Gao, B., & Tsan, M.-F. (2003). Endotoxin contamination in recombinant human heat shock protein 70 (Hsp70) preparation is responsible for the induction of tumor necrosis factor alpha release by murine macrophages. The Journal of Biological Chemistry, 278(1), 174–9. https://doi.org/10.1074/jbc.M208742200