The association of the subunits of component C3 of hagfish complement is unstable and leads to novel degradation during electrophoresis

An opsonic molecule that is designated the third component of hagfish complement (HC3), and a fragment of HC3 known as HC3b have recently been identified in the hagfish, Eptatretus burgeri. These proteins were purified from plasma and generated a set of several bands and/or smears during SDS‐PAGE under standard, non‐reducing conditions. Two‐dimensional electrophoretic analysis of the proteins under non‐reducing and reducing conditions revealed the breakdown of polypeptides at the site of a thioester bond and the concomitant partial release of a split product, depending on the weak covalent or non‐covalent association of polypeptide chains, in a large fraction of molecules of HC3 during SDS‐PAGE. Moreover, the heterogeneity of HC3b can be ascribed to the different configurations of subunits. A similar phenomenon was not observed in the case of lamprey C3, even though breakdown of polypeptides at a thioester bond did occur in some molecules.