The apolipoprotein(a) moiety of lipoprotein(a) interacts with the complement activation fragment iC3b but does not functionally affect C3 activation or degradation.

A previous study has shown that complement component C3 binds to recombinant apolipoprotein(a) (r-apo(a)). In the present report we have investigated the interactions between lipoprotein(a) (Lp(a)), r-apo(a) and C3 in relation to complement activation and degradation. Neither Lp(a) nor r-apo(a) affected complement activation as indicated by sheep and rabbit red blood cell hemolytic assays, and by assessment of the amount of C3a generated in zymosan-activated human serum in the presence or absence of Lp(a). Crossed immunoelectrophoretic analyses indicated that Lp(a) retarded the migration of iC3b in complement-activated serum but had no effects on C3, C3b, C3c or C3dg. Recombinant apo(a) exhibited the same properties as intact Lp(a) indicating that it is the apo(a) portion of Lp(a) that mediates this effect and not the lipid moiety. Low density lipoprotein had no effect on the migration of C3 cleavage fragments. Treatment of Lp(a) or apo(a) with neuraminidase abolished their capacity to alter iC3b migration. SDS-PAGE immunoblotting analysis of C3 activation fragments generated in the presence of Lp(a) demonstrated the usual physiologic C3 cleavage fragments. Rocket intermediate gel immunoelectrophoresis of complement-activated serum demonstrated that Lp(a) did not hinder or accelerate the generation of C3c and C3dg breakdown fragments of iC3b. The results indicate that the apo(a) moiety of Lp(a) alters the migration of iC3b in an electric field but does not affect complement activation or degradation of activated C3. The sialic acid residues on apo(a) are necessary for the apo(a)-iC3b interaction.