A comparison is made between the hemolytic actions of
melittin and the ninth component of
complement (C9).
Melittin and C9 produce "pores" of similar effective radius in erythrocytes under standardized conditions, and their hemolytic action is suppressed by
metal ions at similar concentrations, suggesting a common mechanism. Polyclonal anti-
melittin immunoglobulin G (
IgG) produced in rabbits retards
hemolysis mediated by human C9 in a specific manner. Such
antibodies react in several immunoassays with human and monkey C9 but not with C9 from lower animals, and no inhibition of lysis mediated by C9 molecules from these animals is observed. Thus, it is unlikely that anti-
melittin IgG reacts with a structural
element, such as an amphipathic helix, on human C9 since such structures are also predicted to exist in other C9 molecules. Human C9 and
melittin block cross-reactivity in a dose-dependent manner, and anti-
melittin IgG recognizes an
epitope located between
amino acid residues 245 and 390 of human C9 on "Western" blots. Comparison of the
melittin and human C9 sequences indicates two regions of complete homology, a tetrapeptide at positions 292-295, and a pentapeptide at positions 527-531 in human C9, corresponding to residues 8-16 in
melittin. Inhibition of
hemolysis is not caused by blocking of C9 binding to the
C5b-8 complex; rather the antibody must dissociate from the bound C9 before lysis ensues, indicating that it interferes with a postbinding event. It is proposed that anti-
melittin binds to a conformational
epitope on native, folded human C9 and thereby retards unfolding of the molecule, which is required for membrane insertion and
hemolysis.