Dimeric, trimeric and tetrameric complexes of immunoglobulin G fix complement

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Abstract
The binding of pure dimers, trimers and tetramers of randomly cross-linked non-immune rabbit immunoglobin [Ig] G to the 1st component and subcomponent of the complement system, C1 and C1q, respectively, was studied. These oligomers possessed open linear structures. All 3 oligomers fixed complement with decreasing affinity in the following order: tetramer, trimer, dimer. Complement fixation by dimeric Ig exhibited the strongest concentration-dependence. No clear distinction between a non-cooperative and a cooperative binding mechanism could be achieved, although the steepness of the complement-fixation curves of dimers and trimers was better reflected by the cooperative mechanism. Intrinsic binding constants were about 106 M-1 for dimers, 107 M-1 for trimers and 3 .times. 109 M-1 for tetramers, assuming non-cooperative binding. The data are consistent with a maximum valency of C1 for Ig G protomers in the range 6-18. The binding of dimers to purified C1q was demonstrated by sedimentation-velocity ultracentrifugation. Mild reduction of the complexes by dithioerythritol caused the Ig to revert to the monomeric state (S20, w [Standard Sedimentation Coefficient in Svedbergs] = 6.2-6.5 S) with concomitant loss of complement-fixing ability.

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