Release of phospholipids from complement-mediated lesions on the surface structure of Escherichia coli

When varying numbers of sensitized, 14C-labeled bacteria were treated with a certain amount of complement, in a fixed reaction volume, 14C compounds were liberated into the surrounding medium in proportion to the number of the bacteria, whereas the amount of the phospholipids liberated was constant regardless of the number of the bacteria even in the range of relative excess of complement. Since it is conceivable that a certain amount of complement might form a fixed number of lesions on the surface of all the sensitized bacteria, the amount of the liberated phospholipids seems to be proportional to the number of complement lesions. The 14C-materials released from complement-attacked bacteria were analyzed by isopycnic sucrose density gradient ultracentrifugation and they were mainly free phospholipids and other smaller molecules. A small amount of the smaller membrane proteins were also released as revealed by acid and SDS-polyacrylamide gel electrophoresis. These results suggest that the release of phospholipids is due to the displacement of membrane lipids by the complexes of the late acting complement components during their insertion into the membrane lipid bilayer.