We have examined in detail the relationship between the solubility of the isolated lipid A portion of bacterial lipopoly-saccharides and biological activity. Lipid A isolated by mild acid hydrolysis of the LPS of E. coli 0111:B4, was shown to be an inactivator of complement. This anticomplementary activity was increased over two orders of magnitude after solubilization with triethylamine. In contrast to previously published reports, however, highly active soluble lipid A preparations could be obtained independent of a carrier protein. When such a carrier was present, better than 95% of the carrier did not contribute to the formation of a soluble complex. We conclude that the biological activity of lipid A is highly dependent upon its solubility.
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Morrison et al. (1973) studied this question.