Isolation and characterization of the third component of rat complement.

The third component of complement from rat plasma was isolated by precipitation with polyethyleneglycol in the presence of benzamidine, followed by chromatography of the precipitate on CM-cellulose, hydroxyapatite, and QAE A50-Sephadex, and gel-filtration on Sephadex G-200 superfine. C3 was isolated in its native form, as assessed by its specific functional activity and its immunoelectrophoretic mobility. The recovery of C3 was between 18 and 26%, and the final material was homogenous on SDS-PAGE analysis. Rat C3 has an apparent molecular weight of 187,000 and it is composed of two non-identical disulphide linked polypeptide chains of 123,000 and 76,000. A monospecific antiserum against rat C3 was produced by immunization of rabbits with purified C3. Zymosan treatment of rat serum resulted in cleavage of C3 and in the appearance of component with a greater mobility during immunoelectrophoresis. The plasma concentration of rat C3 in Wistar rats was 581 +/- 49 microgram/ml (mean +/- 1 S.D.). Interaction of highly purified rat C3 with purified human components B, D, and P resulted in the formation of a C3 convertase suggesting compatibility between human and rat C3.