HETEROGENEITY OF C4d and C3d AND THEIR COMPLEX FORMATION WITH SERUM ALBUMIN

High molecular weight (Mr around 175 KD) forms of C4d and C3d as well as free C4d and C3d (Mr about 40 KD) were demonstrable in normal human serum (NHS). Following in vitro C activation in NHS by ΔIgG, the 40 KD C4d component increased markedly. By immunofixation it was hown that the high molecular forms of C4d and C3d reacted with biotinylated anti-human albumin IgG, whereas the 40 KD-free C4d and C3d fragments did not. Furthermore, the incorporation of anti-albumin IgG in the first dimensional gel in crossed Immunoelectrophoresis caused retention of the 175 KD C4d component but not of free C4d. The 175 KD form of C4d had a distinctly higher electrophoretic migration velocity (post-albumin region) than the 40 KD C4d fragment. The C3d-, C4d-serum albumin complexes could not be dissociated by reducing agents (DTT, mercaptoethanol), a non-ionic detergent, or exposure to high and low ionic strength.