REACTION BETWEEN COMPLEMENT SUBCOMPONENT C1Q, IGG COMPLEXES AND POLY-IONIC MOLECULES

Abstract

The strength of the bond between 125I-labeled Clq [q fragment of complement component 1] and immune complexes, Fc piece, dextran sulfate, polyglutamic acid and polylysine was investigated. The binding of Clq to Fc piece, small MW (< 10,000) dextran sulfate, polyglutamic acid and polylysine have values for the functional affinity constant (Ko) in the range 0.2-1.5 .times. 104 M-1. The binding of Clq to immune complexes and large MW polyions (> 100,000) is much greater and lies in the range 3 .times. 107-4 .times. 108 M-1. The differences in the binding constants between the 2 groups can be explained if the Fc piece and small MW compounds bind to only 1 head of the Clq molecule but the immune complexes and large molecules bind to 2 heads. There are probably 6 binding sites on the Clq molecule for dextran sulfate. The enhancement of the binding affinity of Clq by reduction in ionic strength and the reaction with polyions indicate that ionic groups are present near or within the binding sites.