Limited proteolysis of the third component of human complement, C3, by heat treatment.

Human C3 is composed of two disulfide-linked polypeptide chains, termed alpha chain (with a molecular weight of 110,000) and beta chain (with a molecular weight of 75,000). When 3C was heated at above 50 degrees C and at neutral pH, a single peptide bond in the alpha chain was selectively cleaved to yield two alpha chain fragments with molecular weights of 75,000 and 44,000. The two alpha chain fragments and intact beta chain are originally connected by disulfide linkages but are gradually dissociated upon prolonged heat treatment. The dissociation seems to be caused by thiol-disulfide interchange reactions, since the dissociation was prevented by the addition of monoiodoacetic acid and only 1 mol of thiol group was determined to be newly generated upon heat treatment of C3. The heat-induced C3 cleavage reached a plateau when almost 1 mol of thiol group appeared. In addition, the heat-induced C3 cleavage was prevented by pretreatments with C3 convertase and methylamine, which are known to cleave the thioester linkage in the side chain of C3. Thus, the thioester linkage, which is the latent reactive site in C3 and which, upon activation of C3, forms an ester linkage with cell surfaces, seems to make a specific peptide bond extremely heat-labile.