Purification and characterization of the 1q subcomponent of canine complement and its use in the 125I-C1q binding assay for detection of immune complexes.

The complement subcomponent, C1q, was isolated from serum obtained from clinically normal dogs, using a rapid 2-step process involving affinity chromatography. Yield of C1q ranged from 8 to 10 mg/L of serum. Hemolytically active C1q had 3 protein bands after sodium dodecyl sulfate polyacrylamide gel electrophoresis under reducing conditions and formed a single line of identity with rabbit anti-canine C1q. The amino acid composition of canine C1q was similar to that of human C1q and contained a high percentage of glycine. Isolated canine C1q was iodinated, and the fluid-phase binding assay was used to detect circulating immune complexes in dogs with systemic lupus erythematosus and rheumatoid arthritis.