Interaction of radiolabeled endotoxin molecules with human monocyte membranes

Radiolabeled and biologically active endotoxin molecules were prepared, and their binding to monocyte plasma membranes was studied. The binding of 3H-endotoxin and 51Cr-lipid A to isolated membranes was found to be less specific and of lower apparent affinity than that observed using whole cells. Plasma membranes isolated from intact, viable 51Cr-lipid A-pretreated monocytes were found to contain a significant portion of the cell-associated 51Cr-lipid A following Percoll density gradient fractionation of post-nuclear homogenates. When monocytes were pretreated with 3H-endotoxin under the same experimental conditions, all of the label was recovered in the extracellular medium, and subcellular fractionation revealed no fractions which contained tritium. Taken together, our results suggest that specific and high affinity interactions between monocyte membranes and endotoxin molecules are likely to depend on plasma membrane structures which are assembled in intact monocytes but which are disrupted when plasma membranes are isolated from these cells.