Stability of endotoxin detected in human plasma against endotoxin-inactivating factor (EIF): quantitative analysis of EIF using chromogenic endotoxin assay.

Using a quantitative blood endotoxin assay utilizing chromogenic substrate coupled with perchloric acid pretreatment (PCA-LCT), we showed the presence of endotoxin-inactivating factor (EIF) in human plasma in vitro. EIF activity inactivated added endotoxin to about 10(-4) of the initial level within 20 min, followed by a stable phase where the residual endotoxin became resistant to EIF and was not further inactivated. The residual endotoxin may represent the endotoxin in patient plasma which is also EIF resistant. We postulate that endotoxin, upon entering the blood, is rapidly inactivated by chemical modification of its active site, lipid A, through EIF. Subsequently, inactivated endotoxin, mainly consisting of polysaccharide, is gradually removed from circulation by endocytosis in the reticuloendothelial system.