C5b,6 is a product of complement activation of C5 and an intermediate in the formation of the C5b-9 membrane attack complex (MAC) of complement. Each pathway of complement generates proteolytic enzyme complexes (C3/C5 convertases) which are bound to the target surface (Law, S.K.A. and Reid, K.B.M. (1995); Ross, G.D. (1986)). These enzymes cleave a peptide bond in the larger alpha chain of C5 releasing the highly potent anaphylatoxin C5a (74 amino acids) and activating C5b. This is the only proteolytic step in the assembly of the C5b-9 complex. C5b is unstable, but it remains bound to the activating complex for a brief time (~2 min) during which time it either binds a single C6 from the surrounding fluid to form C5b,6 or it decays and aggregates and is no longer capable of binding C6 or of forming C5b-9 complexes. Normally the C5b,6 complex remains bound to the C3/C5 convertase until it binds a single C7 molecule. However, in the absence of the C7 protein, C5b,6 is released into the fluid phase. C5b,6 is a very stable complex. During complement activation some C5b,6 diffuses away from the target cell and may, after combining with C7, enter the membrane of a nearby cell. This is called bystander lysis or “reactive lysis” and can be a significant source of pathology.
Purified C5b,6 can be used to lyse any bilipid membrane surrounding a cell, virus or particle by mixing C5b,6 with the cell in the presence of a compatible source of C7, C8 and C9. Every cell requires a different amount of C5b,6 and conditions necessary for lysis vary widely depending on experimental conditions. Due to the inefficient insertion of C5b,6,7 complexes into membranes from the fluid phase, as compared to those formed directly on the surface, much higher concentrations of fluid phase C5b,6 are required to lyse cells than if complement is activated directly on that cell’s surface.
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