Is the membrane attack complex of complement an enzyme?

Abstract

Recent studies on the functional activities of the membrane attack complex of complement, C5b-9, are reviewed. A new speculative hypothesis has been advanced to account for the ability of complement to mediate lysis of various targets. This hypothesis has three major elements: 1) that the membrane attack complex is an enzyme; 2) that the substrate for this putative enzyme is a membrane constituent; 3) that the substrate specificity of the putative enzyme is dependent on the species source of individual complement components within the C5b-9 complex.

Comparison between complement and melittin hemolysis: anti-melittin antibodies inhibit complement lysis

“…To reconcile the results that anti-melittin antibodies can bind to C9 in solution and must dissociate to allow functional expression, and yet also bind to C9 when associated with a membrane, we propose that two forms of C9 exist on the target membrane after lysis. Earlier experiments by several investigators, notably Borsos and his colleagues [reviewed by Boyle andBorsos (1982) andBoyle (1984)], suggested the existence of lytic and nonlytic or aborted MACs. Thus, when anti-melittin IgGs bind to C9 in solution, they retard expression of the lytic C9 form, possibly by preventing C9 insertion, but later they bind again to C9 molecules that have not unfolded but are associated with the target and may not have inserted.…”