Eighth component of guinea pig complement: purification and characterization

Number of hits necessary for complement-mediated hemolysis

The number of hits necessary for the C8 and C9 steps of immune hemolysis was reexamined with a previously unemployed experimental design, in which various numbers of EAC1-7, excess of the supplementary component and a constant amount of the component tested were incubated in a constant volume (Inoue et al. 1976. Infect. Immun. 13: 337). Our results were consistent with previous findings; the steps of guinea pig C8 and C9, the human C8 each followed a one-hit mechanism, while that of human C9 showed ka multi-hit response. When lysis of sensitized erythrocytes (EA) by normal human serum was analysed in a similar way, one-hit curves were obtained. This result, taken together with the above results, suggests that immune hemolysis occurs by a single lesion including a single C8 and multiple C9 in the case of human complement and that normal human serum contains sufficient excess of C9. On the other hand, when C9-deficient human serum was used for lysis of EA, multiple-hit curves were obtained. The mechanism of lysis by C5b-8 may differ from that by C5b-9.

Interaction of the eighth component of guinea pig complement (C8) with the membrane-bound C5b-7 complex. The binding site of C8 to C5b-7 is formed on association of the alpha-gamma subunit with the beta-chain

The eighth component (c8) of guinea pig complement consists of three polypeptide chains, the alpha-, beta-, and gamma-chains with M.W. of 60,000, 60,000, and 24,000, respectively. The alpha- and gamma-chains are bound by a disulfide bond(s) forming an alpha-gamma subunit, which is linked noncovalently to the beta-chain. The alpha-gamma subunit and the beta-chain were separated and purified from C8 by treatment with sodium dodecyl sulfate (SDS) and gel chromatography on Sephacryl S-300 in the presence of SDS. After removal of SDS, neither alpha-gamma nor beta showed the hemolytic activity of C8 when assayed independently, but showed significant activity in combination, indicating reconstitution of active C8. The recovery of hemolytic activity was 3.48%. When alpha-gamma and beta were incubated successively with EAC-7 with intervening reconstitution of active C8 on the cells was insignificant, irrespective of the order of the reactions. alpha-gamma and beta did not bind to EAC-7 when added separately, but after recombination 7% of alpha-gamma and 9% of beta bound to EAC-7 when EAC-7 was in excess. These results indicate that the binding site of guinea pig C8 to the membrane-bound C5b-7 complex does not exist on either alpha-gamma or beta only but stretches over both or is formed on one subunit after recombination of the subunits.