Table 1.
Comparison of endotoxin content of different apolipoprotein reagents
| source of apolipoprotein | digested (EU/μg) | undigested (EU/μg) | endotoxin in culture medium (ng/ml)1 | inhibited proliferation |
|---|---|---|---|---|
| Company A human plasma ApoA-I | 1.63 | 0.46 | 0.163 | yes |
| Company B E. coli recombinant ApoA-I | >20.00 | -- | >2.000 | yes |
| Company C HEC 293 recombinant ApoA-I | 1.31 | 0.00 | 0.131 | no |
| Company A human plasma ApoA-II | 0.01 | -- | 0.001 | no |
| Company A human plasma ApoA-IV | 0.22 | -- | 0.022 | yes |
total endotoxin when apolipoprotein was added at a concentration of 1 μg/ml of culture medium. Assumes 1 EU = 0.1 ng endotoxin.
Samples were assayed for endotoxin using the Lonza PyroGene™ assay either without or following protease digestion. Protease treatment allows for determination of total endotoxin content including endotoxin whose activity may be masked by associated protein. Under certain conditions, ApoA-I can bind endotoxin and neutralize its activity. Without protease treatment, only endotoxin that is freely available to inhibit macrophage proliferation would be measured. The endotoxin concentrations of the apolipoprotein preparations that inhibited macrophage proliferation were sufficiently high to inhibit macrophage proliferation. Interestingly, Company C ApoA-I did not inhibit macrophage proliferation, presumably because its endotoxin content was completely masked (compare protease-digested and -undigested endotoxin levels).