Tobias P S, Soldau K, Gegner J A, Mintz D, Ulevitch R J
Department of Immunology, Scripps Research Institute, La Jolla, California 92037, USA.
J Biol Chem. 1995 May 5;270(18):10482-8. doi: 10.1074/jbc.270.18.10482.
Endotoxin (lipopolysaccharide; LPS) activates a wide variety of host defense mechanisms. In mammals LPS binding protein (LBP) and CD14 interact with LPS to mediate cellular activation. Using sucrose density gradients 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 to LBP complexes with one molecule of LBP and 1-2 molecules of LPS predominate, while at high ratios of LPS to LBP a large aggregate of LBP and LPS predominates. Complexes of LPS with sCD14 do not form large aggregates, consisting of only 1-2 LPS bound to a single sCD14 even at high multiples of LPS to sCD14. LBP catalyzes LPS binding to sCD14. Catalysis by LBP apparently occurs because LBP provides a pathway for LPS to bind to sCD14 which avoids the necessity for LPS monomers in aqueous solution. The dissociation constants for LPS.LBP and LPS.sCD14 complexes were determined to be 3.5 x 10(-9) and 29 x 10(-9) M, respectively. These numbers suggest that when LBP and sCD14 are present at roughly equal concentrations as they are in normal human plasma and compete for limited LPS, the LPS will predominantly associate with LBP.
内毒素（脂多糖；LPS）可激活多种宿主防御机制。在哺乳动物中，LPS结合蛋白（LBP）和CD14与LPS相互作用以介导细胞活化。我们使用蔗糖密度梯度和一种荧光内毒素衍生物，研究了LPS与LBP及可溶性CD14形式（sCD14）结合的机制。LPS与LBP结合形成两种类型的复合物；在LPS与LBP比例较低时，一分子LBP和1 - 2分子LPS形成的复合物占主导，而在LPS与LBP比例较高时，LBP和LPS的大聚集体占主导。LPS与sCD14的复合物不会形成大聚集体，即使在LPS与sCD14的比例很高时，也仅由1 - 2个LPS分子与单个sCD14结合组成。LBP催化LPS与sCD14结合。LBP的催化作用显然是因为LBP为LPS提供了一条与sCD14结合的途径，从而避免了LPS单体在水溶液中的必要性。LPS·LBP和LPS·sCD14复合物的解离常数分别测定为3.5×10⁻⁹和29×10⁻⁹ M。这些数据表明，当LBP和sCD14以与正常人血浆中大致相等的浓度存在并竞争有限的LPS时，LPS将主要与LBP结合。
