Summary. Initial separation andconcentration ofClq fromfresh, normal equine serumwasaccomplished by precipitation in002Macetate buffer, pH55,at4°for 24h.There-dissolved precipitate wasclarified by centrifugation at80,000 gfor1handthendialysed against Tris-HCl buffer (0-05 M,pH 8-0) containing l0-3 MEDTA.Theclarified dialysate remained biologically active at5°foratleast 4weeks. Biological activity ofequine Clqwasdetermined byassay ofits ability toagglutinate sensitized sheeperythrocytes (EA).Following ammoniumsulphate fractionation, Sepharose 4Bgelfiltration yielded three major peaks. Twoprotein bands weredemonstrated onanalysis of thesecond Sepharose peakbydisc acrylamide electrophoresis, pH83.Elution oftheprotein bands showed EA-agglutinating activity onlyinthebandwhich migrated furthest towardthecathode. Equine Clq isolated bythis method yielded anapproximate fortyfoldpurification inspecific activity. Someproperties ofequine Clqwerecharacterized. Equine Clqwas heat-labile, asshownbyloss ofitsEA-agglutinating activity after heating 580for15min.Moreover, storageat4°andfreeze-thaw cycles greatly reduced EA agglutination. Preliminary determination ofthesedimentation coefficient indicated that itwascomparable tothat reported forhumanandrabbit Clq.
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