The mechanism of action of the C3b inactivator (conglutinogen activating factor) on its naturally occurring substrate, the major fragment of the third component of complement (C3b)

Abstract

Purified 125I labelled human C3b was allowed to react with purified C3b inactivator for various lengths of time, and the products were analyzed by polyacrylamide gel electrophoresis and functional assay. The results indicated that the C3b inactivator proteolytically cleaves C3b in an 2 step reaction. The first cleavage is completed in 3 min, and the product C3b(i) can not activate the alternative pathway. The 2nd cleavage requires 3 hr; and can also be accomplished by small concentrations of trypsin. The product resembles C3c purified from aged serum. Both cleavages occurs at sites on the heavy chains of C3b. (Tonder - Bergen.)