Comparison of Naja n. naja and Naja h. haje Cobra-Venom Factors: Correlation between Binding Affinity for the Fifth Component of Complement and Mediation of its Cleavage

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Two cobra-venom factors, one from Naja n. naja (CVF n), the other from Naja h. haje venom (CVF h), have been purified and compared, functionally and structurally. Both factors interacted with human factors B and xxxD to form a potent C3 convertase, CVFBb. However, while the convertase formed with CVF n did also efficiently cleave C5, CVF hBb had very little C5-cleaving potency only, in particular when human C5 was used as substrate. Studies with agarose-linked CVF preparations indicated that CVF h has only low binding affinity for C5 gp and C5 hu whereas CVF n binds to both C5 species with much higher affinity. Since C5-binding (to CVF or to C3b) is a prerequisite for its cleavage by C3/C5 convertases, the difference in binding potency explains the different C5-cleaving activity of the two CVF preparations. When a ligand for C5, surface-fixed C3b, is present, CVF hBb is also capable of cleaving C5. The difference in activities of CVF n and CVF h is reflected in their different potency to interfere with immune haemolysis and in causing indirect lysis by their complexes with activated factor B. By gel chromatography of the CVF preparations in C5-containing medium, a stoichiometric complex CVF n-C5 (1 + 1) could be demonstrated. An analogous complex of C5 was neither found with CVF h, nor with C3 hu or soluble C3b hu. Structural differences between CVF n and CVF h were revealed by immunodiffusion analysis and by polyacrylamide-gel electrophoresis in presence of SDS. The data available so far provide, however, no clear information about the structure of the C5 binding site.