Human plasmaanti-α-galactoside antibody forms immune complex with autologous lipoprotein(a)

Anti-α-galactoside antibody (anti-Gal) from human plasma that bound to α-galactoside-bearing guar galactomannan gel and was eluted with specific sugar (affinity-purified anti-Gal; APAG) invariably contained apo(a) and apo B subunits in a proportion close to that in plasma lipoprotein(a) [Lp(a)]. Since LDL does not contain apo(a), result suggested Lp(a) as a component of APAG. Lp(a) in APAG was complexed with anti-Gal since plate-coated anti-apo(a) captured Lp(a) along with the antibody. Association of Lp(a) with anti-Gal in APAG was considerably lower in presence of anti-Gal-specific sugar, suggesting that Lp(a) occupied the sugar-binding site of anti-Gal. Content of Lp(a)-bound anti-Gal in APAG, though a minor fraction of total antibody, increased steadily with total Lp(a) content of plasma. Further, Lp(a) released from immune complex-rich fraction of plasma by anti-Gal-specific sugar was proportional to total plasma Lp(a). Anti-Gal titre decreased with increasing Lp(a) concentration among 114 plasma samples. Results indicate the potential of anti-Gal molecules with its binding site partially occupied by Lp(a) molecule(s) to a) use the remaining binding site(s) to recognize other macromolecules or cells and b) transport Lp(a) across Fc receptor-bearing cells.