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Studies of Complement Complex C5b,6 Eluted from—EAC-6: Reaction of C5b,6 with EAC4b,3b and Evidence on the Role of C2a and C3b in the Activation of C5¹

From the Department of Microbiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Abstract

A macromolecular complex containing guinea pig complement fragment C5b (activated C5) and component C6 has been obtained from ,,,,, (sheep erythrocytes carrying antibody and complement components, or fragments, C1, C4b, C2a, C3b, C5b and C6) or from ,,,, by elution. This complex combines with EAC4b,3b to yield an intermediate termed pseudo-EAC-6, which can be lysed by C7, C8 and C9, like genuine EAC-6, but which differs from the latter since it lacks C2a. In contrast, native C5 and C6 will produce EAC-6 only on reaction with ,,. Thus, cell-bound C2a is required for the reaction with native C5 but is not needed when C5 is supplied in the activated and complexed state C5b,6. Further, anti-C3 blocks the reaction of EAC4b,3b with C5b,6. These results suggest that in the reaction with native C5 the cell-bound C2a mediates the enzymatic activation of C5, whereas the cell-bound C3b supplies a binding site. In addition, the present experiments show that the activated state of C5b is stabilized by complexing with C6.

Footnotes

¹ This work was supported in part by United States Public Health Service Research Grant 5RO1-AI-02566-13, United States Public Health Service Training Grant 5 TO1-AI-00282-08, and National Science Foundation Grant GB7406X1.

² Recipient of a postdoctoral fellowship under National Institutes of Health Training Grant 5 TO1-AI-00282-08. Present address of M. B. Goldlust: Squibb Institute for Medical Research, Department of Biochemical Pharmacology, Princeton, New Jersey 08540.

³ Recipient of Research Career Development Award 6KO4 GM50193-03 from the National Institute of General Medical Sciences.

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