Binding of Desoxycholate, Phosphatidylcholine Vesicles, Lipoprotein and of the S-Protein to Complexes of Terminal Complement Components

Abstract

Quantitative measurements of sodium desoxycholate (DOC) binding to intermediate complexes of the terminal components showed increasing binding ability with the progression of complex assembly. C5b-6 and C5b-7 bound, respectively, 24 and 27 moles DOC/mole complex, C5b-8 and C5b-9 bound 63 and 86 moles, DOC/mole complex, respectively. All four complexes behaved as amphiphilic proteins upon charge shift electrophoresis. DOC above the critical micellar concentration prevented aggregation of nascent C5b-7 (C5b-7*), dissociated the S-protein from SC5b-9, and efficiently extracted C5b-9 from complement-treated sheep erythrocyte membranes without dissociating the complex. Nonionic detergents did not bind to the four complexes, were incapable of preventing aggregation of C5b-7*, and of dissociating the S-protein from SC5b-9. The forming C5b-9 complex, depending on the experimental conditions, was found to combine with either DOC, S-protein, lipoprotein, or lipid vesicles. In absence of either of these substances it aggregated with itself. The binding and aggregation phenomena are considered expressions of the same underlying molecular reaction, which is also considered responsible for the membranolytic function of the complex. Efficiency of binding of C5b-7* varies 1000-fold depending on the properties of the acceptor. Reversible ionic interaction of C5b-6 with the acceptor before the reaction with C7 may be responsible for this observation. Reversible ionic interaction of C5b-6 with DOC micelles before hydrophobic binding of the detergent may also explain the differential binding of ionic and nonionic detergents. It is proposed that the C5b-7 complex overcomes the charge barrier of the membrane by ionic interactions and associates itself with the membrane by secondary hydrophobic interactions. Subsequent binding of C8 and C9 leads to increased hydrophobicity of the complex, causing a perturbation of the membrane by transmembrane channel formation or by the generation of protein-phospholipid micelles.