Physico-chemical characteristics of the third and fourth component of complement after dissociation from complement-cell complexes

Abstract

The radioactivity labelled third (C′₃) and fourth (C′₄) components of human complement were dissociated from erythrocyte membrane—complement complexes. The differential effectiveness of various reagents in causing dissociation suggests that C′₃ and C′₄ are bound to membrane sites by hydrophobic bonds. Density gradient ultracentrifugation of the dissociated components showed that they are bound in macromolecular form. Their electrophoretic mobility was greater than that of their native counterparts. In physical and immunochemical properties, dissociated C′₃ and C′₄ closely resembled the respective haemolytically inactive conversion products that, during immune haemolysis, accumulate in the fluid phase. It is concluded that in immune haemolysis C′₃ and C′₄ become linked to membrane sites only following modification by their respective converting enzymes.