LIPOPOLYSACCHARIDE-BINDING PROTEIN-MEDIATED COMPLEXATION OF LIPOPOLYSACCHARIDE WITH SOLUBLE CD14

Endotoxin (lipopolysaccharide; LPS) activates a wide variety of host d efense mechanisms. In mammals LPS binding protein (LBP) and CD14 inter act with LPS to mediate cellular activation, Using sucrose density gra dients and a fluorescent endotoxin derivative we have investigated the mechanism of LPS binding to LBP and the soluble form of CD14 (sCD14). LPS binds to LBP to form two types of complex; at low ratios of LPS t o LBP complexes with one molecule of LBP and 1-2 molecules of LPS pred ominate, while at high ratios of LPS to LBP a large aggregate of LBP a nd LPS predominates, Complexes of LPS with sCD14 do not form large agg regates, consisting of only 1-2 LPS bound to a single sCD14 even at hi gh multiples of LPS to sCD14. LBP catalyzes LPS binding to sCD14, Cata lysis by LBP apparently occurs because LBP provides a pathway for LPS to bind to sCD14 which avoids the necessity for LPS monomers in aqueou s solution, The dissociation constants for LPS . LBP and LPS . sCD14 c omplexes were determined to be 3.5 x 10(-9) and 29 x 10(-9) M, respect ively. These numbers suggest that when LBP and sCD14 are present at ro ughly equal concentrations as they are in normal human plasma and comp ete for limited LPS, the LPS will predominantly associate with LBP.