Initiation of C3 cleavage in the alternative complement pathway.

The capacity of C3 and 125I-C3 to interact with B and D to generate a C3 convertase was recognized by identification of C3b and Bb on immunoelectrophoresis and 125I-C3b on disc gel electrophoresis. Since the C3 was devoid of C3b as assessed by immunoelectrophoresis, alkaline disc gel electrophoresis, and isoelectric focusing, the initial C3 cleavage was not by the C3b-dependent C3 convertase. In addition, on isoelectric focusing C3 hemolytic activity and the capacity to permit B cleavage by D were isoelectric at pH 6.4, WHEREAS THE CAPACITY OF C3b to permit cleavage by D was isoelectric at pH 5.65. Comparison of the dose-response effects of C3b and C3 for D revealed linear and sigmoidal relationships, respectively, consistent with formation of an initial C3 convertase independent of C3b followed by generation of the C3b-dependent convertase in the reaction initiated with native C3. Further, preincubation of C3 with C3bINA did not diminish its subsequent capacity to permit B inactivation by D as compared to the introduction of C3bINA during the assay, thus supporting the view that native C3, B, and D can form a convertase capable of generating initial C3b.