The Binding Properties of Human Complement Component Clq

Quantitative measurements have been made of the interaction of human complement subcomponent C l q with mucopolysaccharides. The binding of Clq to heparin was quantitatively examined by utilizing an assay that employs a ‘‘%labeled low molecular weight heparin glycosaminoglycan (LMW-Hep) (Mr = 8500). Two classes of binding sites were detected. The first class of sites bound 2.02 mol of LMW-Hep/mol of Clq with a Kd of 76.6 ma. The second class of sites complexes with 12 mol of LMW-Hep with a Kd of 1.01 pM. The higher affinity-binding site for LMW-Hep could be assigned to the collagenous region of Clq (Clq-c); 2.2 mol of 1251LMW-Hep were bound/mol of purified isolated Clq-c with a Kd = 381 nM. In contrast, the isolated clq globular region did not bind to ‘251-LMW-Hep. The binding of LMW-Hep to Clq and the Clq-c region was confirmed by fluorescence polarization experiments; C l q and Clq-c bound 2.3 and 2.02 mol of fluorescamine-labeled LMW-Hep/mol of protein, respectively. A variety of mucopolysaccharides were able to inhibit interaction of Clq with 1251-LMW-Hep, the most effective being heparan sulfate and dermatan sulfate. LMW-Hep (2.5 11~) inhibited the ability of C l q (0.5 m) to recombine with ClF (1.4 m) and ClS (1.6 m) to form hemolytically active Ci . At 250 mM, LMW-Hep inhibited the hemolytic activity of reconstituted Ci . The ability of mucopolysaccharides to interact with purified Clq suggests a role for such molecules in the regulation of the first component of complement.