Alternate Complement Pathway: Factors Involved in Cobra Venom Factor (CoVF) Activation of the Third Component of Complement (C3)

Abstract

An immunochemically pure plasma protein, β₂II, possessed properdin factor B activity. On the other hand, purification of this protein was associated with loss of the ability to interact with cobra venom factor (CoVF) so as to inactivate the third complement component (C3) or activate the terminal complement sequence as assessed by the lysis of unsensitized erythrocytes. C3 inactivation was restored, and terminal component activation partially restored, by combining with β₂II and CoVF a 35,000 molecular weight protein isolated from euglobulin, termed factor D. An additional factor of approximately 160,000 m.w., termed factor E, also obtained from euglobulin, was required for complete reconstitution of CoVF-induced terminal component activation. Addition of radioiodinated CoVF to serum, EDTA-plasma, or serum rendered deficient in β₂II, resulted in an apparent increase in the molecular weight of CoVF from 144,000 to 220,000 indicating that the interaction of CoVF with a binding protein can occur independently of magnesium ions or β₂II. Complexing of CoVF was not seen with β₂II, factor D, or a mixture of the two but was observed with preparations of factor E.