Activation of Terminal Components of Human Complement by a Trypsin-Activated Complex of Human Factor B and Cobra Venom Factor

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Abstract
Cleavage of C3 [3rd complement component] by CVF-.hivin.B [trypsin activated complex of human factor B and cobra venom factor] was demonstrated by hemolytic, immunoelectrophoretic and immune adherence reactions. No cleavage of C5 was detected by immunoelectrophoresis, but C5 hemolytic activity, assayed with E[erythrocytes]A[antibody] .**GRAPHIC**. decreased although less than C3 hemolytic activity. The co-existence of C3 with limiting amounts of C5 did not reduce the final degree of hemolysis of guinea pig erythrocytes (GPE) induced by late-acting components C6 through C9 and CVF-.hivin.B. Thus a CVF-.hivin.B hemolytic system composed of GPE, C5 through C9 and CVF-.hivin.B provided a method for titration of terminal components of human C. CVF-.hivin.B was able to generate hemolytically active sites of .**GRAPHIC**. on GPE by activation of C5, C6 and C7. The complex .**GRAPHIC**. in the fluid-phase decayed within 1 min but .**GRAPHIC**. on GPE was quite stable. Originally insensitive sheep erythrocytes became sensitive to the CVF-.hivin.B hemolytic system if C3b sites were present, suggesting that cell-bound C3b played a role in orienting the positions of .**GRAPHIC**. to be fixed. CVF-.hivin.B could be recovered quantitatively from the supernatant of the reaction mixture in which the hemolytically active intermediate .**GRAPHIC**. was formed through the interaction between C5 to C8 and CVF-.hivin.B.