The role of HDL consisting of SP-40,40, apo A-I, and lipids in the formation of SMAC of complement.

Soluble membrane attack complex (SMAC, SC5b-9) of complement contained apolipoprotein A-I (apo A-I) and lipids. Since SC5b-9 contained no apolipoprotein A-II (apo A-II), the incorporation of apo A-I and lipids into SC5b-9 was not caused by a non-specific binding of high-density lipoprotein (HDL). The incorporation of apo A-I into SC5b-9 was confirmed by a double agar diffusion immuno-assay. SC5b-9 contained one molecule of apo A-I and 3.5% lipids by weight. The composition of the lipids was phospholipid: total cholesterol: triglyceride = 47.5:45.0:7.5, and the ratio of free cholesterol:esterified cholesterol was 22.3:77.7. Since SC5b-7 contained apo A-I and SC5b-9 free of SP-40,40 contained no apo A-I, the HDL consisting of SP-40,40, apo A-I, and lipids was incorporated into SMAC at the stage of SC5b-7 via SP-40,40 as a binding site.