Role of C1 in the complement activating effect of Pseudomonas aeruginosa lipopolysaccharide preparations

The complement consumption of endotoxin preparations extracted by the trichloroacetic acid or phenol-water method from different Pseudomonas aeruginosa strains was measured in normal human and guinea pig serum and in serum chelated with Mg2+-EGTA. In the chelated serum, which was essentially Ca2+-free, the first component of complement (C1) could not exert its function. All preparations tested consumed considerably less complement activity in chelated than in normal serum. The proportion of CH50 units fixed in Mg2+-EGTA and in normal serum was always higher in the tricholoracetic acid extract than in the phenol-water extract of the same strain. The part of LPS molecule that was able to activate the complement system in Ca2+-free serum was partially separated from the C1-requiring part by the combination of different extraction methods. The results suggest that on the LPS molecules two different sites are responsible for the complement activating effect through the classic and the alternative pathways.