The role of C3 proactivator (C3PA) and a factor isolated from cobra venom (CoF) in the formation of a principle able to cleave C3 was investigated. The results clearly demonstrate two modes of interaction of C3PA with CoF. In isolated form, C3PA and CoF were found to form a reversible protein-protein complex in free solution. This complex had some C3 cleaving activity. In the presence of minute amounts of a partially purified normal serum substance, factor D, the C3PA-CoF complex was stabilized and its efficiency in cleaving C3 was greatly increased. Factor D is thus an activator of C3PA. A firm complex composed of C3PA and CoF, and possessing C3 cleaving activity, was also formed by addition of CoF to serum.

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