Studies on the mode of action of the fifth, sixth and seventh component of human complement in immune haemolysis

Abstract

The sixth (C′₆), seventh (C′₇) and eighth (C′₈) components of human complement were separated from each other by chromatography on hydroxyl apatite and obtained in functionally pure form. Density gradient ultracentrifugation revealed that C′₆ and C′₇ interact with the fifth component of complement (C′₅) to form a reversible protein—protein complex. Evidence was obtained that this complex occurs also in unfractionated, fresh human serum. Kinetic analysis of the reaction sequence in immune haemolysis disclosed that C′₅, C′₆ and C′₇ act, in this order, subsequent to the third component (C′₃) and prior to C′₈. Exploration of the mode of action of C′₅, C′₆ and C′₇ led to the formulation of two alternative hypotheses. The three components act either sequentially or as one functional unit. Their fluid phase interaction, the shape of dose—response curves and the encountered difficulty to isolate intermediate reaction products favour the concept that they act as a functional unit.