Characterization of the soluble membrane attack complex (SMAC) of carp (Cyprinus carpio) complement

The membrane attack complex (MAC) of mammalian complement, which is composed of complement components, C5b, C6, C7, C8 and C9, shows cytolytic activity by disturbing the structure of cytoplasmic mambrane. The soluble form of the membrane attack complex, termed SMAC, of the complement system is innert unlike the membrane attack complex formed on target cell membranes. Thus formation of SMAC is inferred as a regulatory mechanism of the cytotoxic activity of the complement system. In the present study, a complex homologous to mammalian SMAC was purified from zymosan-activated carp serum by gel filtration through Sepharose CL-6B and affinity chromatography using anti-carp C9-Toyopearl. The purified carp SMAC showed a partial antigenicity of C9 and behaved as a 1,020 kDa α -globulin. Two-dimensional SDS-PAGE of carp SMAC revealed polypeptide spots of C5b, C6, C7, C8 and C9, in agreement with those previously identified in carp MAC. In addition, three spots specific to SMAC were detected on the two-demensional gel, which may represent proteins responsible for solubilization of MAC, such as SP40,40 and S-protein present in mammalian SMAC. These results suggest that structure and function of SMAC have been conserved through the evolution of vertebrates.