Isolation and analysis of the mechanism of action of an inactivator of C4b in normal human serum

Abstract

A complement regulatory principle, C4b inactivator, was isolated in a partially purified form from normal human serum. The C4b inactivator, a β1 globulin with an approximate molecular weight of 88,000 daltons, and which may be identical to C3b inactivator, cleaved C4b in free solution or on the surface of cells, and rendered it unable to participate in hemolytic reactions or to interact with cells having receptors for C4b. C4b inactivator functioned by cleaving the α' polypeptide chain of C4b at a single site which was sufficient to dissociate the molecule into 2 fragments, C4c and C4d, and to inactivate its biological function. Certain structural correlates of C4 functions deriving from these studies are discussed, and a model for C4 structure based on these findings is presented.